They are twelve Krabby Patties on wheat buns. Although not seen when SpongeBob makes the patties, they apparently contain salt, as suggested by John when he asks for extra quantities of it for his order of these patties.
Role in episode
These are what John orders at the beginning of the episode. Conforming to the name of the burgers, SpongeBob serves them in a farmer-like style.
John requests extra salt for the burgers, but Squidward lousily refuses to fulfill his request.
Although they contain lettuce, it is not mentioned when SpongeBob narrates the ingredients of the patty.