Preheat the oven to 400 degrees F. Pulse the hazelnuts and rosemary in a food processor until the nuts are about the size of whole peppercorns. Transfer to a large plate. Combine the mustard and horseradish together in a small bowl.
Rub the oil all over the steak and season with salt and pepper, to taste. Heat a cast iron or heavy-bottomed pan over medium-high heat until very hot, about 5 minutes. Lay the steak in the pan and sear without moving until each side becomes a rich brown color, about 3 minutes a side.
Transfer to a cutting board and brush the mustard mixture all over the steak. Dredge the steak in the hazelnuts, pressing so they adhere to the meat. Arrange the steak on a rack set over a baking sheet or roasting pan. Put into the oven and cook until an instant-read thermometer inserted into the thickest part of the steak registers 125 degrees F for medium rare, about 20 minutes. Transfer the meat to a cutting board and let rest for 10 minutes. Slice the steak and arrange it on a serving platter. Serve.