Cheesesteak Stuffed Peppers

Bahan-bahan (8 porsi)

• 2 g reen bell peppers (halved)
• 8 slices provolone cheese
• 1 onion (sliced)
• 2 cups mushrooms (sliced, optional)
• 1 tablespoon butter
• 6 -8 ounces deli roast beef (thinly sliced)