Making Philly Cheesesteak Stuffed Peppers is easy if you follow these steps:

1. Preheat Oven: Preheat your oven to 375°F (190°C).
2. Prepare Peppers: Cut the tops off the bell peppers and remove the seeds. Place them upright in a baking dish.
3. Cook Beef: In a large skillet, heat olive oil over medium heat. Add the chopped onion and cook until translucent. Then, add the sliced beef and season with garlic powder, onion powder, salt, and pepper. Cook until the beef is browned.
4. Add Mushrooms: If using, add the sliced mushrooms to the skillet and cook for an additional 3-4 minutes until tender.
5. Mix Cheese: Remove the skillet from heat and stir in half of the provolone and mozzarella cheeses until melted and combined.
6. Stuff Peppers: Carefully fill each bell pepper with the beef and cheese mixture, pressing down gently to pack it in.
7. Top with Cheese: Sprinkle the remaining cheeses on top of each stuffed pepper.
8. Cover and Bake: Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, until the peppers are tender and the cheese is bubbly and golden.
9. Garnish: Once done, remove the stuffed peppers from the oven and let them cool slightly. Garnish with chopped fresh parsley before serving.

These steps will guide you through creating the amazing Philly Cheesesteak Stuffed Peppers effortlessly.