Saute onion in butter until translucent. Add chicken breasts and cook until chicken is just done (juices run clear). Remove and set in a large baking dish. In a large measuring cup, mix soup (10 oz can, Campbells) and milk and stir to remove the lumps. Pour over chicken in baking dish. Sprinkle with parmesan cheese and bake for 15 - 20 minutes at 325F. Saute mushrooms in butter. When chicken is removed from oven, spread mushrooms over top and serve. Serve with mashed potatoes or rice. Excellent with cauliflower or broccoli as veggie. There is sufficient cheese sauce to add over the veggies. This meal can be made a very fast preparation by microwaving the soup/milk mixture til warm and then adding to chicken/onions in the frypan. Cover and simmer for 5 minutes, then garnish and serve.