The best Steak Sandwich

Yield: 4 sandwiches

Ingredients

1	(12- ounce) 1- inch thick steak
	Kosher salt
	Freshly ground black pepper
2 ½	tablespoons Olive oil
2	yellow onions, sliced in rings
½	teaspoon fresh thyme leaves
¾	cup good mayonnaise
1	tablespoon Dijon mustard
1	tablespoon whole-grain mustard
2	tablespoons sour cream
⅛	teaspoon salt
4	sub rolls
½	cup baby arugula

Preparation

Step 1	Season the steak with salt and pepper on both sides. Heat 1 tablespoon of olive oil in a pan over high heat until it's almost smoking, then sear the steak on each side for 1 minute. Reduce the heat to low and cook the steak until desired doneness. Remove from heat and let sit for 10 minutes. Slice the steak into strips.
Step 2	Using the same pan, heat 1 1/2 tablespoons of olive oil over medium heat. Add the onion and thyme and saute for 10 minutes, until the onions arecaramelized, stirring occasionally.
Step 3	Whisk mayo, mustards, sour cream, and salt into a bowl and set aside.
Step 4	To assemble the sandwiches, spread a tablespoon of mayo sauce on each side of the roll.Place a layer the steak strips on top of the mayo, sprinkle with saltand pepper, and top with the caramelized onion rings. Place the baby arugula on top of the onion rings, and cover the sandwiches with the top half of the buns.