Cut the leftover steak into thin slices.

Heat oil in a large non-stick skillet over medium heat.

Add onions and cook for about 10 minutes or until softened, stirring frequently. Transfer to a plate.

Top the bottom half of bread with arugula and caramelized onions.

Place a layer of the thin steak slices on top, and spread mustard on top.

Season with optional salt and pepper if necessary.

Cover the sandwiches with the top half of the bread.