Saute apple and onion in butter or margarine until tender. Add curry powder and saute 1 minute more. Add flour and continue to cook 1 minute longer. Add stock or broth and milk; stir well.
Salt and pepper chicken breasts to taste and lay in a single layer in a 9x13 inch (or larger) baking dish. Pour sauce mixture over chicken breasts and bake in preheated oven for 45 to 50 minutes, until chicken is done.
