In a large skillet, cook beef and sausage over medium heat until no longer pink; drain. Stir in soup and tomatoes; set aside.
Place half of the potatoes and onion into a greased 13x9-in. baking dish. Layer with remaining potatoes and onion. Top with mozzarella cheese and meat mixture.
Cover and bake at 350° for 1 hour. Uncover; sprinkle with Gruyere cheese. Bake 10-15 minutes longer or until potatoes are tender and cheese is melted. Let stand 10 minutes before serving. Sprinkle with parsley.