Cut the peppers in half and remove the seeds.
Bake the empty peppers for about 10 minutes to soften slightly.
Heat olive oil in a skillet.
Cook the onion and mushrooms until tender.
Add the beef and cook until browned.
Season with garlic powder, salt, pepper, and Worcestershire sauce.
Fill each pepper with the beef mixture.
Cover generously with cheese.
Bake until the peppers are tender and the cheese is melted.