Heat 1 tablespoon oil in a skillet over medium heat. Add the sausages and cook, turning, until browned on all sides and cooked through, about 15 minutes.
Meanwhile, heat the remaining oil in another skillet over medium heat. Add the peppers, onion, salt, and pepper. Cook until soft and golden, 10 to 12 minutes.
Toast rolls if desired. Halve the sausages lengthwise. Place 2 halves on the bottom of each roll and top with the pepper-onion mixture.
Cover with another roll half, if desired. Serve the mustard on the side.