MAKES 4 SANDWICHES
EACH SANDWICH About 714 calories, 47g protein, 40g carbohydrate, 40g total fat (15g saturated), 3g fiber, 150mg cholesterol, 1,444mg sodium
Heat 1 tablespoon oil in a large skillet over medium heat. Add the onions, season with 1⁄4 teaspoon each salt and pepper, and cook, covered, stirring occasionally, for 8 minutes. Uncover and cook, stirring occasionally, until golden brown and very tender, 6 to 8 minutes more; transfer to a bowl.
Meanwhile, slice the steak very thinly.
Wipe out the skillet and heat the remaining 1 tablespoon oil over medium-high heat. Season the steak with 1⁄2 teaspoon each salt and pepper and cook, stirring occasionally, until browned and cooked through, 3 to 4 minutes.
Lay the cheese over the steak, cover, and cook until the cheese melts, about 2 minutes. Remove from the heat and gently fold the cheese into the steak.
Form sandwiches with the rolls, steak mixture, and onions. Top with mushrooms or bell peppers, if desired.