Preheat your oven to 375°F (190°C). Arrange the halved bell peppers in a baking dish, cut side up. Bake for 10 minutes to slightly soften them.

Heat olive oil in a large skillet over medium heat. Add the diced onion, green bell pepper, and mushrooms (if using). Cook for 5–6 minutes until softened.

Add the ground beef or steak slices to the skillet. Cook until the meat is browned and cooked through, breaking it up if using ground beef. Drain excess grease if necessary.

Stir in the garlic powder, Worcestershire sauce, salt, and pepper. Cook for 1–2 more minutes to combine flavors.

Remove the pre-baked bell peppers from the oven. Fill each pepper with the beef and vegetable mixture. Top with a slice of provolone cheese or a generous sprinkle of shredded mozzarella.

Return the stuffed peppers to the oven and bake for 15–20 minutes, or until the cheese is melted and bubbly.

Serve hot, garnished with chopped parsley if desired. Pair with a fresh salad or roasted vegetables for a complete meal.