In a large skillet, heat the olive oil over medium-high heat. Add the sliced flank steak, season with salt and pepper, and cook for about 3-4 minutes until browned. Remove the steak from the skillet and set aside.
In the same skillet, add the sliced onion, mushrooms, and bell pepper. Sauté for about 5-7 minutes until the vegetables are softened and caramelized.
Return the cooked steak to the skillet with the vegetables and mix well. Cook for an additional 2-3 minutes to heat through.
Preheat your oven broiler. Slice the hoagie rolls in half lengthwise and place them on a baking sheet.
Divide the steak and vegetable mixture evenly among the hoagie rolls. Top each with provolone cheese.
Place the baking sheet under the broiler for 2-3 minutes, or until the cheese is melted and bubbly. Keep a close eye to prevent burning.
Remove from the oven and serve immediately, adding hot sauce or mustard if desired.