Heat oven to 375°F. In a skillet over medium-high heat, cook onion and mushrooms in olive oil with a pinch of salt until softened, 6 to 8 minutes. Add steak, garlic powder, pepper, and Worcestershire; sear 2 to 3 minutes until just browned. Combine and remove from heat.

Place pepper halves cut-side up in a baking dish. Spoon filling into each pepper, then drizzle any pan juices over the top.

Cover dish with foil and bake 20 minutes. Uncover, top each pepper with a provolone slice, and bake uncovered 10 to 15 minutes more until cheese is melted and peppers are tender.

Let rest a couple of minutes, then transfer to plates and serve hot.