Cook 1/4 cup finely chopped onion in 1 tablespoon olive oil until softened. Stir in 1/2 pound ground beef and 2 tablespoons Worcestershire sauce; season with salt and pepper and cook until barely pink. Fill 16 white stuffer mushrooms. Top each with a 2-inch square of sliced cheddar cheese. Bake in a 375 degrees oven for 15 minutes. Dollop with ketchup.