01 - Preheat oven to 400°F. Arrange bell pepper halves in a baking dish, cut side up.
02 - Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add sliced onions and mushrooms; sauté until softened and lightly golden, about 5 minutes. Transfer to a bowl.
03 - Add remaining olive oil to the skillet. Add beef and cook until just browned, about 2-3 minutes. Add garlic, Worcestershire sauce, salt, and pepper; stir well.
04 - Return onions and mushrooms to the skillet. Mix to combine and cook for 1-2 minutes. Remove from heat.
05 - Place a half slice of provolone cheese in the bottom of each pepper half. Fill each pepper with the beef mixture, then top with another slice of provolone.
06 - Cover baking dish with foil and bake for 20 minutes. Remove foil and bake an additional 5-10 minutes, until cheese is bubbly and peppers are tender.
07 - Let cool slightly before serving.