While playing the game, I constantly wondered why some food was the hot type and others were regular. Even food right next to each other. Then by complete accident, I blew up a guy standing next to food and it turned into hot food! Afterwards I started experimenting with different weapons. The list below is what I have confirmed so far.

What you have to do:
-Shoot any food (monk meal or priest porrage) with an explosive weapon. From what I can tell as long as the blast touches the food, it'll transform.

-Why waste ammo doing this? Well if you are in a pinch and low on health, shooting the food essentially doubles the health restore. This is especially helpful during boss fights.

Monk Meal (small bowl):

• Cold - About 8 health restored
  
• Hot - About 16 health restored

Priest Porrage (large bowl):

• Cold - About 20 health restored
  
• Hot - About 40 health restored

*I am playing on Hard mode with no cheat codes. The health restored may be different on other difficulty settings. I assume this will work on other difficulty settings as well.

Confirmed:

• Bazooka
  
• Firebomb
  
• Split Missle
  
• Drunk Missle
  
• Heat Seeker
  
• FlameWall Secondary fire**
  
• Excalibat Secondary fire**

Confirmed doesn't work:

• Bullet weapons
  
• God Hand
  
• Dog Mode
  
• Dark Staff**

Unconfirmed:

• Other difficulty modes besides Hard

**I tested these with cheats enabled in order to complete the guide quickly.

I will update the guide as I find and test the weapons. Feel free to contribute in the comments.