Preheat your oven to 400°F.
Arrange the bell pepper halves on a baking sheet, cut side up, and bake for 15 minutes until they start to soften.
While the peppers are baking, heat olive oil in a large skillet over medium heat.
Add the sliced onions and mushrooms to the skillet, sautéing until they are tender and golden, about 5-7 minutes.
Season the steak slices with salt and pepper, then add them to the skillet. Cook until the steak is just browned, about 3-4 minutes.
Remove the skillet from heat. Spoon the steak, onions, and mushrooms evenly into each bell pepper half.
Top each stuffed pepper with a slice of provolone cheese.
Return the peppers to the oven and bake for another 10 minutes, or until the cheese is bubbly and golden.
Serve warm.