I LOVE Stuffed mushrooms. They are great and can be filled with so many different things. These are one of a few variations of these beefy like vegetables that I will post and all can be baked at the same time and put out for guests for Thanksgiving and Christmas. If you have left overs, feel free to use that as well. These do make a lot of mushrooms if several kinds are made but leftovers can be stored in the fridge as a snack for the next day, or you can scale back the recipe.
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 mushrooms with beef and onion mix. Top each with a 2-inch square of sliced cheddar cheese.
Bake in a 375° oven for 15 minutes. Remove and let cool. Dollop with ketchup.