In a skillet, saute onion in 1 teaspoon butter until tender. Add sausage, noodles, mushrooms, soups and milk; mix well. Transfer to a 13-in. x 9-in. baking dish coated with cooking spray. Sprinkle with cracker crumbs; dot with remaining butter.
Bake, uncovered, at 375° for 20-25 minutes or until heated through.