Preheat your oven to 375°F (190°C). Slice the tops off the bell peppers and scoop out the seeds and membranes. Place them in a baking dish.

Heat olive oil in a large skillet over medium-high heat. Toss in the onions and mushrooms, cooking until soft and golden, about 5 minutes. Add the steak, garlic powder, salt, and pepper. Sauté until the steak is just cooked through, about 3-4 minutes.

Line each pepper with a slice of provolone cheese. Fill the peppers with the steak mixture, then top each with another slice of cheese.

Pour a splash of water into the bottom of the baking dish (to help steam the peppers). Cover with foil and bake for 20 minutes. Remove the foil and bake another 5 minutes, until the cheese is bubbly and golden.

Let cool for a few minutes, then serve warm—with a sprinkle of parsley if you like!