Prepare the Steak: Season the 1 pound of flank steak generously with salt and pepper on both sides. This will enhance its natural flavors.
Heat the Oil: In a large skillet over medium-high heat, add 2 tablespoons of olive oil. Let the oil heat for a minute until shimmering.
Cook the Steak: Add the seasoned flank steak into the skillet. Cook for about 4-5 minutes per side for medium-rare, or longer if you desire a more well-done steak. Transfer the cooked steak to a plate and let it rest for about 5 minutes before slicing.
Caramelize the Onions: In the same skillet, add the thinly sliced onion. Stir occasionally and cook for about 10-15 minutes, or until the onions are golden brown and caramelized. Season with salt to taste.
Slice the Steak: After resting, slice the flank steak against the grain into thin strips.
Assemble the Sandwich: Preheat your broiler. Cut the baguettes lengthwise and lay them on a baking sheet. Layer the sliced steak, followed by the caramelized onions, and top with 2 slices of provolone cheese on each baguette.
Bake: Place the baking sheet under the broiler for 2-3 minutes, or until the cheese is melted and bubbly.
Serve: Remove from the oven, close the baguettes, slice into manageable portions, and serve immediately.