Seasoning the steak generously with salt and pepper.

Add 1 tablespoon of oil to a large cast-iron skillet over high heat. Once shimmering, adding the steak.

Fry the first side for 2 minutes and then flip over.

Fry the other side for 3 more minutes until the steak is nicely browned on the outside and still medium rare in the middle. Remove from heat and let it rest for 5 minutes. Then cut it into thin slices.

Turn the heat to medium and add the rest of the oil to the skillet.

Add onions and cook for about 9-10 minutes or until golden and softened, stirring frequently. Transfer to a plate and set aside.

Lightly butter the bread and toast until golden.

Place the bottom half of the bread on a plate and top with arugula and caramelized onions.

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

Cover the sandwiches with the top half of the bread. Serve immediately or pack for lunch.