Step 1

Season the steak with salt and pepper on both sides.

Step 2

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.

Step 3

Remove from heat and let sit for 10 minutes. Slice the steak into strips.

Step 4

Using the same pan, heat 1 1/2 tablespoons of olive oil over medium heat.

Step 5

Add the onion and thyme and saute for 10 minutes, until the onions are caramelized, stirring occasionally.

Step 6

Whisk mayo, mustards, sour cream, and salt into a bowl and set aside.

Step 7

To assemble the sandwiches, spread a tablespoon of mayo sauce on each side of the roll.

Step 8

Place a layer the steak strips on top of the mayo, sprinkle with salt and pepper, and top with the caramelized onion rings.

Step 9

Place the baby arugula on top of the onion rings, and cover the sandwiches with the top half of the buns.