Repurpose leftover steak in this cheesesteak-inspired sandwich.
Preheat broiler with rack 6 inches from heat. In a large skillet, heat oil over medium-high. Add onion and peppers; cook, stirring often, until softened and beginning to char, 10 minutes. Season with salt and pepper. Remove from heat; stir in vinegar and steak.
Place bottom halves of rolls on a large rimmed baking sheet; dividing evenly, top each with meat mixture, then cheese slices. Broil until cheese has melted, about 2 minutes. Remove from oven; transfer to a cutting board.
If you can't get thinly sliced pepper Jack cheese at the deli counter, buy it coarsely grated from the dairy aisle instead.