Remove casings and fry sausage in large skillet, stirring to keep meat crumbly. Remove sausage from pan and drain off fat. Melt butter in skillet. Add onion and mushrooms and saute until onion is tender but not browned. Combine beaten eggs, milk, sausage, onion, mushrooms, tomatoes, cheese, salts, and pepper. Mix well and pour into a greased 13 x 9 baking pan. Bake at 400 degrees 30 to 40 minutes, or until knife inserted in center comes out clean. (Salt is optional) If you have some spaghetti sauce, heat it up and serve along side! Good recipe for a brunch.