Preheat your oven to 400 degrees F. Cut the tops off the bell peppers and remove the seeds. Place them cut-side up in a baking dish and bake for 15 minutes to soften them slightly.
While the peppers soften, heat olive oil in a large skillet over medium-high heat. Add the sliced onion and sliced mushrooms. Cook for 6 to 7 minutes, stirring occasionally, until the onions are caramelized and the mushrooms are golden.
Push the onions and mushrooms to the side. Add the shaved steak to the skillet and season with salt, black pepper, and garlic powder. Cook for 3 to 4 minutes, stirring to break it apart, until the steak is browned.
Add the Worcestershire sauce and toss everything together. Stir-fry for 1 more minute to let the flavors combine.
Remove the peppers from the oven. Spoon the steak, onion, and mushroom filling generously into each pepper, packing it in tightly.
Place a slice of provolone cheese on top of each stuffed pepper. Return to the oven and bake for 10 minutes until the cheese is melted, bubbly, and lightly golden. Serve hot.