Get some roast turkey from the deli counter (Boar’s Head all natural preferred, but just dont get any heavily seasoned kind), sliced as thin as possible, and sneak him a little bit every now and then. Hopefully she wont think much of it sitting in the fridge, and you can even make yourself a sandwich with it once while shes around so she really doesnt suspect anything.
My cats love their daily turkey, and it keeps them from overeating the dryfood that I free-feed them. They only eat the dry food when hungry cause they know they get their “kitten chicken” every morning 😺
Chicken is fine too.