How to Make It
Put the steak on a work surface and, using a Jaccard knife or a fork, poke the steak all over until it's well tenderized. Season the steak with 1 tablespoon of salt and 1 teaspoon of pepper. Let stand at room temperature for 45 minutes.
Meanwhile, light a gas grill. In a medium bowl, whisk the sour cream with the horseradish and season with 2 teaspoons of salt and 1 teaspoon of pepper.
Oil the grill grate. Pat the steak dry and brush it lightly with oil. Grill over high heat, turning once, until lightly charred on both sides, about 6 minutes. Continue to grill the steak over moderate heat, turning occasionally, until an instant-read thermometer inserted in the center registers 120°, about 5 minutes longer. Transfer the steak to a carving board and let rest for 10 minutes. Thinly slice the steak against the grain and serve with the horseradish sauce.