Mushroom, Onion and Cheese Casserole Mushroom, Onion and Cheese Casserole Sue Ferachi Sept. 04, 2013 Comments Mushroom, Onion and Cheese Casserole Serves 6 to 8. Recipe is provided by Sue Ferachi. 1 (103/4-oz.) can cream of mushroom soup Milk equal to 1 soup can 6 sweet onions, thinly sliced 1 (8-oz.) pkg. fresh sliced mushrooms 1 (8-oz.) pkg. shredded mild cheddar cheese 1. Mix soup and milk in bowl. Layer onions and mushrooms in a 9-inch-by-11-inch baking dish. Pour soup-milk mixture over onions and mushrooms. 2. Sprinkle with cheese. Bake at 350 degrees until cheese is melted and onions are tender, about 30 minutes.