Good question and one you need to know!

Each dish has its own set of recipe cards, for example, the burger dish has burger recipe cards, the bruschetta has bruschetta recipe cards etc..Your dishes are classified in different categories depending on how rare your dishes and ingredients are: -

“Common” recipe cards are gray and can be found in the gray crates

“Gourmet” recipe cards are purple and can be found in the yellow and purple crates.

The Merchants will also offer you recipe cards that you can buy directly from them.

Once you have collected enough recipe cards to improve your dish you need to head to your Academy's cookbook to level up your dishes. 

You can only collect recipe cards for dishes that you have unlocked in the game.