Cook 1/4 cup finely chopped onion in 1 Tbs olive oil until softened. Stir in 1/2 lb ground beef & 2 Tbs Worcestershire sauce; season with salt & pepper & cook until barely pink. Fill 16 white stuffer mushrooms. Top each with 2 inch square piece sliced cheddar cheese. Bake in 375 degree oven for 15 minutes. Dallop with ketchup.