How to Make It
Split baguette lengthwise. Scoop out and discard soft inside of baguette (reserve and freeze for bread crumbs), leaving a ½-inch crust. Place cut side up on a baking sheet.
Preheat broiler to high with oven rack 6 inches from heat. Season both sides of flank steak with ½ teaspoon each salt and black pepper. Heat oil in a large cast-iron skillet over high; add steak and cook, turning once, until deeply browned on both sides, about 4 minutes per side for medium-rare. Transfer to a cutting board and let rest 10 minutes before thinly slicing.
Add bell pepper and onion to skillet and cook, stirring, until tender, 6 to 8 minutes. Season with remaining ½ teaspoon salt and ¼ teaspoon black pepper.
Top baguette halves with vegetables, steak, and cheese. Broil until cheese is melted and bubbly, about 1 minute. Cut into pieces before serving.