Combine ground beef with salt and pepper and shape into 11/2” balls. Meanwhile, cook onions with garlic in skillet containing combined butter and oil. Brown onions then cover and cook until almost tender. Add mushrooms and cook lightly. Remove onions and mushrooms from skillet and set aside. Brown meatballs well on all sides. Drain off excess drippings, stir in remaining ingredients. Return onions and mushrooms to pan. Cover and cool until meatballs are done.