There are options

If you are feeding your cat a dry prescription diet, you can purchase a few cans of it to use as a small treat. If you don't like giving them the canned food because it is wet or you normally feed canned and want something different, then bake the canned food and make your own prescription cookies. If the diet you are feeding is a limited ingredient diet like a duck and pea formula, you can get some fresh duck and give them small amounts of the plain meat. If you are feeding a hydrolyzed protein diet then you should stick only to the prescribed food because even these formulas break down the proteins to be unrecognizable by the immune system and fresh food will not have that protection. If you don't know which type you are feeding just check the bag- it should say on there if it is hydrolyzed protein. Good luck!