The Sandwich is a food that Mr. Bumpy eats in the Bump in the Night episode "Night of the Living Bread".
In the episode, Mr. Bumpy is pretending to be a mad scientist while being assisted by Squishington. Bumpy then says that he is going to create something that hasn't been created before, Squish believes that Bumpy is going to create life, while Bumpy doesn't want to do that. Bumpy powers his creation with eletricity conducted by a kite. Mr. Bumpy removes the cloth away, revealing his creation is a giant sandwich.
Bumpy licks his lips hungrily and lifts the sandwich above his head, the top piece of bread falls off and disappears behind the microwave. Bumpy notices the top piece is gone, but isn't concerned about it and devours the whole sandwich. While Bumpy and Squishington leave, the missing slice of bread mutates and comes to life.