Cut the peppers in half, top to bottom, and remove any seeds. Brush them with 1 tablespoon of olive oil and place them, cut side down, in a 9x13-inch baking dish. Bake for 20 minutes.
Meanwhile, slice the steak across the grain very thinly. Season with ¼ teaspoon salt and black pepper to taste.
In a large skillet, heat the remaining 1 tablespoon of olive oil over medium-high heat. Once shimmering, add half of the beef and cook 2 minutes without stirring. Stir and cook 1 minute more. Repeat with the remaining beef. Transfer the beef to a bowl and set aside.
Add the onion to the skillet, adding a bit more oil if needed. Cook for 3 to 4 minutes or until it begins to soften. Add the mushrooms and cook until tender, about 6 minutes. Stir in the butter, steak, Worcestershire sauce, steak seasoning, and minced garlic. Cook for 30 seconds more, stirring to combine.
Flip over the baked peppers. Stuff the peppers with the steak mixture and top with cheese.
Bake uncovered for 15 to 20 minutes or until the peppers are tender.
Notes
Freeze the steak for 30 minutes to make slicing easier, and always cut against the grain for extra tenderness.
Prep the stuffed peppers up to two days in advance, cover, and refrigerate. Add cheese before baking.
Or bake, cool, and freeze on a baking sheet, then transfer to zippered bags for up to one month. Thaw overnight before baking.