Step 1

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

Step 2

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

Step 3

Cut the leftover steak into thin slices.

Step 4

Lightly butter the bread and toast until golden.

Step 5

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

Step 6

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

Step 7

Season with optional salt and pepper if necessary.

Step 8

Cover the sandwiches with the top half of the bread.