Preheat oven to 325°. Prepare Creamy Wild Rice Soup Mix as directed on package with water. While soup is simmering, sauté onion and mushrooms in butter until onion is clear. Add all remaining ingredients to soup. Pour mixture into greased 2 1/2-quart baking dish. Bake at 325° for 1 hour or until casserole reaches desired consistency. Makes 4-5 servings.