In a medium skillet over medium heat, cook hamburger and diced onion. Cook until meat is brown and onions are soft. Season as desired with salt and pepper.
Add the optional mushrooms and cook until warm.
Drain any fat, then add taco seasoning according to package directions.
Prepare the tomatoes by slicing the top off to remove any stems and/or leaves.
Flip tomatoes so they are cut side down and slice six wedges in each, being careful to not cut completely through the tomato.
Divide taco meat evenly among the tomatoes, then top with cheese sauce.
Bake in a 400 degree oven for 20-25 minutes, or until tomatoes are wilted and browned slightly.
Remove from oven, add desired toppings, and enjoy!