Heat broiler. Season the steak with ½ teaspoon each salt and pepper. Line a rimmed baking sheet with foil and place the steak on one end.
Toss the tomatoes and onion in a medium bowl with the oil, vinegar, ½ teaspoon salt, and ¼ teaspoon pepper.
Place the tomatoes, cut-side down, and the onion on the other end of the baking sheet.
Broil, turning everything once, until the steak reaches the desired doneness, 4 minutes per side for medium-rare.
Transfer the steak to a plate and return the vegetables to oven until browned, about 1 minute. Slice the steak.
Place the bottom halves of the bread on plates and top with the steak, vegetables, cheese, and remaining bread. Serve warm.