Saute crumbled sausage until no longer pink. Drain and put aside in bowl. Saute onion and mushrooms in butter until soft but not brown. Stir into sausage. Blend in remaining ingredients and mix well. Pour into greased 9 x 13 inch pan and bake at 400 degrees for 30 to 35 minutes or until knife inserted in center comes out clean. Serves 8 to 10.