The Ultimate Steak Sandwich | Chef Josh Capon

In a skillet, cook onions and garlic in olive oil over medium heat until caramelized, about 15 minutes. Season with salt and pepper.

Combine both mustards, the mayonnaise, salt, pepper and a splash of red wine vinegar in a small bowl. (Dijonnaise can be made in advance and refrigerated for a few days.)

Preheat oven to 400˚F. Season the steak with salt and pepper on both sides.

Heat a cast iron skillet over high heat until hot. Add enough oil to lightly coat the bottom and heat until hot. Add the steak and cook until browned on both sides, then place in the oven until an instant-read thermometer registers 125˚F to 135˚F for medium-rare, about 15 minutes.  

Remove from the pan to a cutting board and let rest for 5 to 8 minutes before thinly slicing. Turn on the broiler.

Drizzle olive oil over ciabatta and season with salt and pepper. Toast the bread. Spread Dijonnaise on both sides of the bread, place sliced steak and cheese on the bread. Place under the broiler to melt cheese, then remove. 

In a mixing bowl, toss caramelized onions and garlic with the arugula, 2 tablespoons red wine vinegar, and salt and pepper. Add dressed arugula to the sandwich and serve.