The general gist of the food mechanic is easy add two ingredients to make a recipe, the two additional slots provide stats boosts but do not change the resulting recipe except for the secret recipes which have a set 3rd ingredient needed to work.

To get ingredients you can complete the related tasks that can be seen when hovering over the missing ingredient, you can get them randomly assigned as reward for a quest and you can get it from the Hag Food Merchant.

Note: the ingredient and recipe parts of this guide is just, *ahem* aquired, from this marvelously typed up source:
https://www.pcinvasion.com/total-war-warhammer-ii-groms-cauldron-recipes-ingredients-and-food-merchant-guide/

The challenges have either been discovered by me or been crowdsourced from the steam forums.

There are five types of ingredients and they’re all color-coded. You can use this as a clue when looking at the Grom’s Cauldron recipes panel. Additionally, the ingredients that you place inside Grom’s Cauldron will provide their buffs.

Beast Blitz (orange)

Troll Meat
Available by default
“Regeneration” passive for Goblin and Night Goblin infantry units

Lion Meat
Win a battle against an army with lions, lion chariots, lion hybrids, or lion mounts (ie. High Elves – White Lions of Chrace)
+10 speed for Goblin cavalry, Goblin chariots, and Squig Herd units; +12 charge bonus for Squig units

Lizard Legs
Win a battle against an army with giant, dinosaur-like creatures or mounts (ie. Lizardmen – Saurus Oldblood)
+15 armor and +8 leadership for Goblin cavalry, Goblin chariots, and Squig Herd units

Boar Meat
Equip a character with a War Boar mount (ie. Orc Warboss lord)
+10 charge bonus and +10% weapon strength for Orc and Big ‘Uns cavalry and chariot units

Gobbo Legs
Obtain a trophy from a successful Call to Waaagh!
“Berserk” passive for Orc infantry units (all armies)

Mucky Stuff (yellow)

Centigor Milk
Win a battle against an army with six-limbed, centaur-like creatures
“Immense Power” passive for Rogue Idols; +10% research rate

Spider Egg Yolk
Recruit an Anachronok Spider
Adds five Lava Spider Archers units to your Regiments of Renown pool; “Maternal Instinct” ability for Anachronok Spiders (spawn Spiderlings during battles)

Ectoplasm
Defeat an army with ghostly apparitions (ie. Vampire Counts – Syreens)
“Ghostly Protection” passive for Rogue Idols; -20% casualties suffered from all types of attrition

Monster Goo
Recruit Da Swamp Fings Regiment of Renown (requires a character to hit level 16 first) “Monstrous Disease” passive for Trolls and Giants; +2 recruit rank for Troll units

Stunty Ale
Sack a Dwarf settlement (Karak Zorn is your best bet since it’s close by, otherwise, you’ll end up having to travel all the way to the Spine of Sotek region in Lustria just to find more Dwarfs)
“Dulled Senses” passive for Trolls and Giants; -50% recruitment cost for Troll units

Fishy Chunks (blue)

These ingredients can be randomly rewarded when you check out sunken treasures at sea. Alternatively, you can obtain them if you sack race-specific settlements.


Goldfish
Sack an Empire settlement
+10% income from all buildings; -1 global recruitment duration; +8 obedience (public order) in all provinces

Tentacles
Sack a Norscan settlement.
+10% income from sacking and looting settlements; enables frostbite attacks for all characters

Hard Shell Crab
Sack a Vampire Coast settlement
+10 scrap; +12% physical resistance for Goblin and Night Goblin units; +10% income from raiding

Ocean Clams
Sack a Bretonnian settlement
Gain a random amount of gold; +10% casualty replenishment rate

Pufferfish
Win a battle against an army with a Bloated Corpse (ie. Vampire Coast)
Adds five Soopa-Squig units (exploders) to your Regiments of Renown pool; “Spiky Skin” passive for Goblin and Night Goblin infantry units

Sporez n’ Shroomz (red)

Green Sporez
Automatically unlocked as your second ingredient when you complete the first step for Blacktoof’s Head in a Jar. The requirement is to have 50 scrap at the start of your turn.
+5 reputation (for the Call to Waaagh! mechanic); +10 growth in all provinces

Indigo Milk Caps
Reach level 15 with an Orc Shaman
-25% hero action cost; +10% hero action success chance

Stinky Fungi
Reach level 15 with a Giant River Troll Hag
-8 enemy leadership; -5 enemy siege holdout time (minimum 1 turn)

Glowing ‘Shrooms
Reach level 15 with a Goblin Great Shaman or Night Goblin Shaman
-10% spell cooldown; +10 starting Winds of Magic amount

Hell Pepper ‘Shrooms
Recruit any Night Goblin Fanatic unit (requires Black Orc Barracks from Musta Fieldz building chain)
“Frenzy” passive for Goblin and Night Goblin units; +25% fire resistance

Flyin’ Gubbinz (green)

Dragon Tail
Win a battle against an army with any Dragon, Wyvern, or Frost Wyrm creature, including mounts (ie. High Elf – Moon Dragon)
“Dragon Breath” ammunition for Goblin and Night Goblin missile units; projectiles will cause explosions

Harpy Head
Sack a Dark Elf settlement
+10% ammunition for Goblin and Night Goblin missile units
“Harpy Teeth” ammunition for Goblin and Night Goblin missile units; increased number of projectiles

Eagle Talons
Retrieve the Axe of Grom
“Eagle Sight” ammunition for Goblin and Night Goblin missile units; increased armor-piercing damage

Winged Leather
Win a battle against an army with a flying creature from Lustria or winged machinery, including mounts (ie. Lizardmen – Ripperdactyl)
+10% range for Goblin and Night Goblin missile units
“Hollow Bone” ammunition for Goblin and Night Goblin missile units; causes anti-large damage

Phoenix Claws
Obtain a Call to Waaagh! trophy against any Elf faction
“Phoenix Fire” ammunition for Goblin and Night Goblin missile units; causes fire damage

Recipes

You have to combine two main ingredients in order to create a unique recipe.
Only one ingredient per “color” is allowed — ie. red + orange or red + blue.

Note 1: The more new recipes get cooked, the stronger Grom becomes. He’ll gain bonuses to his HP, leadership for Goblin units, and additional capacity for Goblin Big Bosses. Lastly, once you’ve cooked all 15 recipes, Grom gets the “Perfect Vigor” attribute.

Note 2: As mentioned, you only need to take note of the color-coded ingredients to create a majority of recipes. Since we already listed the ingredient-specific buffs, we’ll focus on the recipe-specific buffs for Grom.


The Magnum Pizza
Orange + yellow
+15% physical resistance and +15 melee defense (Grom only)

Grom’s Surf & Turf
Orange + blue
+10% casualty replenishment rate for Goblin units and +10% casualty replenishment (Grom’s army)

Shroom ‘n’ Offal Sausage
Orange + red
“Bangers & Smash” ability and +25 charge bonus (Grom only)

Misty Mountain Broth
Orange + green
Enables sundering attacks and +100 base weapon damage (Grom only)

Gobbo Seafood Quiche
Yellow + blue
“Goggly Glare” ability and “Terror” attribute (Grom only)

Cheesy ‘Shrooms
Yellow + red
“Slime Smoothie” ability and “Stalk” attribute (Grom only)

Nuggets ‘n’ Slime
Yellow + green
+20% campaign line-of-sight, +10% campaign movement range, and immunity to attrition (Grom’s army)

Fishy ‘Shroom Burgers
Blue + red
“Fishy Breath” ability and “Perfect Vigor” attribute (Grom only)

Bony Meat Skewers
Blue + green
“Spiky Skin” passive and +30% missile resistance (Grom only)

Wyvern Stew
Red + green
“Unbreakable” attribute and “Frenzy” passive (Grom only)

Secret recipes


There are color combinations for the main ingredients, sure, but you’ll also need to add an extra ingredient once you unlock additional slots. We’ll get to that in a while, but, for now, let’s look at the secret recipes:


Fried Squirrel Fish
Green + blue, Spider Egg Yolk as secret ingredient
Enables lighting strike battles when reinforcements are present (Grom only);
15% ambush success chance (all armies)

Elven Foie Gras
Red + orange, Eagle Talons as secret ingredient
+4 recruit rank for all units;
-2 turns global recruitment;
+50 unit experience gain per turn (faction-wide)

Bugman’s Battered Ribs
Red + orange, Stunty Ale as secret ingredient
+15% missile strength for artillery units;
+15% ammunition for artillery units;
“Flash Bomb” ability (Grom’s army)

Mork’s Blood Pudding
Yellow + orange, Hell Pepper ‘Shrooms as secret ingredient
“Fiery Convocation” spell;
Enables flammable attacks;
Enables flaming attacks (Grom only)

Ghost Crab Royale
Blue + orange, Ectoplasm as secret ingredient
“Fear” attribute for Goblins and Night Goblins (all armies);
+10% ward save (Grom only)

Note: this list isn't exhaustive and some challenges do have multiple solutions. I have also by no means tested all of these.

"Mushy Meat" =
- Nuggets n' Slime (YELLOW + GREEN)

“Four-legged land-beasts pizza; no cold-blood meats.”
- The Magnum Pizza (YELLOW + ORANGE)

"Dish wot got big bones in so I's can pick ma teef afta" =
- Bony meat skewers (BLUE + GREEN)
Thanks to Kilth

"Me wants sumfink with sporez or 'shroomz!" =
- Mork's blood pudding (ORANGE + YELLOW, HELL PEPPER 'SHROOMS as 3rd ingredient.)
Thanks to Teramilkloli

Me only likes da ´richest´quiche! Da ones wot bring ya moneys!
Assuming just any Gobbo Seafood Quiche will do (YELLOW + BLUE), *except* Pufferfish since it doesn't give a money buff.
Thanks to Kilth

"Me want burger! Must be stinky or spicy!"
Any BLUE + STINKY FUNGUS (RED). Assuming HELL PEPPER 'SHROOMS will also work for "Spicy".
Thanks to Kilth

"Stinky burger"
- (BLUE + RED, made sure red used was STINKY FUNGI)
Thanks to Sarin

"Me loves good pie! Any kind of pie!"
The magnum pizza (YELLOW + ORANGE)
Thanks to Warstooge

Me want sumfink slurpy n' soupy! add 'ollow bones for crunch!
Misty mountain Broth made with wing leather
ORANGE + WING LEATHER (green, must be this! = the 'ollow bones)
Thanks to Real_Joker

"Me wants sumfink with beast bits! A dish wot needs three fings in it!"
- Bugman's Battered Ribs (RED + ORANGE + STUNTY ALE)
Thanks to Yiunko
-Elven Foie Gras (RED + ORANGE + EAGLE TALONS)
Thanks to Gamma115

"Me wants sumfink with mucky stuff! A dish wot needs three fings in it!"
Bugmans battered ribs (RED + ORANGE + STUNTY ALE as 3rd ingdredient)
Thanks to Kilth

“Me wants sumfink with fishy chunks. A dish wot needs three fings in it!"” =
- Ghost Crab Royale (ORANGE + BLUE, ECTOPLASM as 3rd ingredient)
thanks to RED Floyd /(Weber)\
- Fried Squirrel Fish secret recipe (GREEN + BLUE, SPIDER EGG YOLK as 3rd ingredient.)

"Me want something with flyin' gubbinz! A dish wot needs three fings in it""
- Fried Squirrel Fish (BLUE + GREEN + SPIDER EGG YOLK)
Thanks to Dennis