Preheat oven to 225 degrees. Prepare medium baking dish with non-stick spray.
Place beef slices, one or two at a time (your preference), but cover the entire bottom of the dish.
Wash chicken breasts and place on top of beef slices. DO NOT SEASON...soup has sufficient seasoning.
Spread UNDILUTED soup over chicken breasts. Cover dish with aluminum foil.
Bake 4 hours....Do absolutely nothing! Go shopping, take a nap, read a book, but don't worry about the chicken. Each breast piece is a serving, so make what you need, increasing ingredients to accommodate guest count.