*Use the 12 slices if you want it extra cheesy.
Preheat oven to 350 degrees F.
Melt 2 tbs of the butter and spread in a 13 x 9-inch baking pan.
Prepare the chicken breasts by cutting them in half lengthwise to make 2 thinner breast pieces.
Lay the chicken breasts across the bottom of the pan to the entire bottom is covered.
Distribute mushrooms (if using) across the chicken breasts)
Place the cheese slices on top of the chicken/mushrooms, covering them completely
In a small bowl, mix the soup and the milk. Pour over top of the casserole then spread out so everything is covered.
Melt the remaining 4 tbs of butter.
Distribute stuffing evenly across the top of the casserole; drizzle with the 4 tbs melted butter.
Bake at 350 degrees F for 45-60 minutes, or until chicken is done and the top of the casserole is bubbling.