Maldon sea salt or other flaky sea salt, for garnish
Instructions
For The Creamy Horseradish Sauce:
In a medium bowl, whisk heavy cream until thickened and soft peaks form. Fold in the crème fraîche or sour cream, the prepared horseradish, chives, lemon zest, lemon juice, grainy Dijon mustard, salt, and pepper. Refrigerate for at least 30 minutes before using, or transfer to an airtight container and refrigerate up to 3 days.
For The Butter-Basted Steak:
Pat the steak dry with paper towels, then season liberally on all sides with salt (we like to use one teaspoon or kosher salt for each pound). Let the steak rest at room temperature for 1 hour before cooking it, or refrigerate it loosely, covered, up to 2 days. (If putting the steak in the refrigerator, bring it to room temperature for at least 30 minutes (and no longer than 2 hours) before cooking.)
Heat the oil over high heat in a large (at least 12-inch) heavy-bottomed cast-iron skillet until just beginning to smoke. Carefully add the steak and cook. Using tongs, flip the steak every 15 to 20 seconds, until a pale golden-brown crust starts to develop, about 4 to 5 minutes total (you want it to be around 100 to 110°F before you add the butter).
Reduce the heat to medium and add butter, herbs, and garlic. Continue to cook, flipping the steak frequently and basting any light spots with foaming butter. Tilt the pan slightly so the butter collects at one side to baste.
Use a spoon to pick up butter and pour it over the steak, aiming at light spots. Continue flipping and basting until an instant-read thermometer inserted into the thickest part of the steak reads 120° - 125°F, about 6 to 8 minutes total. Immediately transfer the steak to a rimmed baking sheet and pour the pan juices on top. Immediately transfer the steak to a rimmed baking sheet and pour the pan juices on top. Let it rest for 10 minutes until the internal temperature is 145°F for medium. Carve and serve.