What about butter or cream (not milk)? I have found they are long in the tooth when the meat is too lean. Also, I find tuna unappealing for that reason. But agreed, cats are picky eaters. One of mine only would eat cooked chicken, and would beg it off my plate. I would give her a large piece of breastmeat, and then she would eat it in the corner, leaving me in peace.

Also, the dry food for cats is like junk food for humans. Full of carbs.

What about butter?