In a wok or large skillet, stir-fry cubed beef, onion, garlic, salt and pepper in oil for 3 to 5 minutes. Remove from pan and keep warm. In the same pan, stir-fry asparagus and mushrooms in butter until asparagus is tender. Return beef mixture to pan. Stir in soy sauce and lemon juice; heat through. Serve with rice. Yields 4 servings.