Preheat oven to 350 degrees F. In a large bowl, combine beef, mushrooms, vegetable soup mix, bread crumbs, milk and egg until well combined.
Form into two loaves, 7 x 5-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 and 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.