Place the loaf of bread on a cutting board with a chopstick on either long side of the loaf. Make 9 equal slices crosswise through the loaf, using the chopsticks to keep from cutting all the way through.
Cut around the sides of the bell peppers, discarding the centers with the ribs and seeds, then thinly slice.
Season the flank steak all over with salt and pepper.
Thinly slice the steak at a 45-degree angle into ½-inch (1 ¼ cm) thick pieces.
Heat the oil in a large cast-iron skillet over high heat. Cook the steak for 4 minutes on each side, or until a nice brown crust forms. Remove the steak from the pan and set aside.
Preheat the oven to 350°F (180°C). Line a baking sheet with parchment paper.
Add the onion to the pan and cook for 5 minutes until it starts to soften and brown on the edges.
Move the onions to one side of the pan and add the peppers to the other side. Season with salt and pepper to taste. Cook the peppers and onion for another 5 minutes, or until caramelized. Remove the pan from the heat.
Set the sliced loaf of bread on the prepared baking sheet.
Fold 9 provolone slices in half and place in between the slices of bread. Place 2 pieces of flank steak in every other slit. Place peppers and onion in the slits with the steak. Place the remaining 4 slices of provolone over the loaf of bread.
Bake for 20 minutes, or until the cheese is browned.
Pull apart the sandwich at the slits with just cheese.
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