Place pepper halves cut side down on baking sheet. Place in oven for 20 minutes.
Meanwhile, in large skillet heat oil. Add onion, mushrooms, and garlic to pan. Sautee until onions are clear and mushrooms tender.
Season beef with salt and pepper. Add to mix and cook through. This will depend on how thick your beef is cut.
If meat and vegetables mixture is done before peppers are ready, remove from heat and keep warm.
Remove peppers from oven and flip over. Turn on broiler. Stuff with meat mixture and top with cheese. Return to oven for two minutes or until cheese is browned. serve hot.