Back in the olden days the classic go-to for sick cats was one-ingredient pureed chicken in a glass jar from the human babyfood aisle. It no longer exists, so I have had to cook & puree it myself.

You'd think now that people are more aware of allergies, etc. that there would be more 1-ingredient babyfoods available, not fewer. Adding one new food at a time is the best way to learn baby's tolerance for various things. Rather than just giving drugs for bouts of diarrhea, you knew to avoid or limit food X and give more of food Y.