Seared Steak and Arugula Sourdough Sandwich

• olive oil · extra virgin1 tbsp
• garlic · 2 cloves, lightly crushed2 cloves
• beef sirloin steak · about 1 inch thick12 oz
• red onion · very thinly sliced1 medium onion
• unsalted butter · unsalted3 tsp
• dijon mustard1 1/2 tsp
• mayonnaise · olive oil or avocado oil based, no seed oil3 tbsp
• salt · kosher, divided1 tsp
• black pepper · freshly cracked3/4 tsp
• lemon juice · fresh1 oz
• sourdough bread · cut into 4 thick slices6 oz
• arugula · washed and dried2 oz
• horseradish · prepared, drained1 oz

Steps

1. 1Pat {{sirloin steak}} dry on both sides. Season all over with {{salt}} and {{black pepper}} and let it sit at room temperature while you prep everything else.
2. 2Stir together {{mayonnaise}}, {{horseradish}}, and {{dijon mustard}} in a small bowl until smooth. Set aside.
3. 3Slice {{sourdough bread}} into four thick slices. Toast them in a dry skillet or toaster until the surface is crisp and lightly browned.
4. 4Heat {{olive oil}} in a heavy skillet over high heat until it shimmers. Lay in the steak and sear without moving it for about 3 minutes, then flip and sear the other side. Add {{butter}} and {{garlic}} to the pan, tilt it, and spoon the foaming butter over the steak until it reaches medium rare.
5. 5Move the steak to a board and let it rest so the juices settle. Slice it thin against the grain and toss the slices in any juices left on the board.
6. 6Toss {{arugula}} with {{lemon juice}} and a small pinch of {{salt}} just before building the sandwich so it stays lively.
7. 7Spread the horseradish mixture on the cut sides of the toast. Layer on the sliced steak, {{red onion}}, then the dressed {{arugula}}. Close each sandwich, press gently, and cut in half.