Steak Sandwich with Grilled Onions

This Steak Sandwich with Grilled Onions is restaurant-level goodness piled high on ciabatta bread with a tangy mustard-mayo sauce.

Stir all ingredients for the sauce together in a small bowl and set it aside until needed.
⅓ cup mayonnaise, 1 tablespoon whole-grain mustard, 2 tablespoons Dijon mustard, 2 tablespoons sour cream, ⅛ teaspoon kosher salt

Season the steak liberally with salt and pepper on both sides.
12 ounces New York strip steak, kosher salt, black pepper
Heat the olive oil in a medium skillet over high heat until it's almost smoking.
1 tablespoon olive oil
Add the seasoned steak to the hot pan and sear on each side for about a minute.
Reduce the heat to low and cook the steak for about 7 to 10 minutes for rare, and up to 15 minutes for well done.
When the cooking time is finished, remove it to a plate and cover tightly with foil. Allow the steak to rest for 10 minutes while you cook the onions.

Using the same pan without cleaning it, heat the remaining olive oil over medium heat.
1 1/2 tablespoons oive oil
Add the onion and thyme and saute, stirring often, for about 10 minutes or until the onions are brown and caramelized.
2 yellow onions, ½ teaspoon fresh thyme leaves

Slice the steak on an angle across the grain.
Spread the mustard-mayo sauce on top and bottom of the bread.
2 ciabatta buns
Place a layer of steak strips on the bottom of the bun, sprinkle with salt and pepper, and top with the caramelized onions.
Place baby arugula on top of the onions and cover the sandwich with the top half of the bun.
½ cup baby arugula