MEATLOAF: Preheat oven to 350°F. In a large bowl, combine beef, mushrooms, vegetable soup mix, bread crumbs, milk and egg until well combined. Form into two loaves, 7X5-inches each. Place loaves in a shallow baking pan with rack. Bake for 45 minutes. MUSHROOM SAUCE: In a small bowl, combine milk and cornstarch; set aside. In a large skillet, melt butter over high heat. Add mushrooms and onion; cook and stir over moderate heat until mushrooms are golden, about 10 minutes. Stir in beef broth and 1/2 cup water; bring to a boil. Stir cornstarch mixture into mushroom mixture in skillet. Cook over high heat, stirring constantly, until slightly thickened, about 1 minute.