Cut the flank steak across the grain into ¼-inch strips. Season generously with salt and pepper.

Heat the oil in a large skillet over medium-high heat. Add the steak and cook, stirring frequently, until it is browned, about 5-6 minutes. Transfer from the skillet to a plate.

Add the peppers and onions and cook, stirring frequently, until they are crisp-tender, about 4 minutes. Add the beef and barbecue sauce to the skillet and cook, stirring occasionally, until heated through, about 2-3 minutes.