Preheat oven to 200 C / 180 C fan. Halve the peppers lengthways and deseed then place in a large baking dish and bake for 20 minutes to soften.
Heat the oil in a large frying pan set over medium-high heat, add the onions and mushrooms and season with salt and pepper. Cook until soft, 6 minutes. Add steak and season with more salt and pepper. Cook, stirring occasionally, 3 minutes. Stir in Italian seasoning.
Add provolone to bottom of baked peppers and top with steak mixture. Top with another piece of provolone and grill until golden, 3 minutes.