Heat 1 tablespoon of the oil in a skillet over medium heat. Add the sausages and cook, turning, until browned on all sides and cooked through, about 15 minutes.
Heat the remaining oil in another skillet over medium heat. Add the peppers and onion. Cook until softened and golden, 10 to 12 minutes. Season with the salt and pepper. Halve the sausages lengthwise and place 2 halves on the bottom of each roll. Top with some of the pepper-onion mixture and, if desired, the top of the roll. Serve with the mustard on the side.
Tip: To save several minutes of cooking time, halve each sausage link lengthwise before adding it to the skillet.