Rinse beef and pat dry. In a small bowl, mix 1/4 cup salt and sugar. Rub mixture all over beef, set in a rimmed pan, cover, and chill 3 to 4 hours. Rinse meat and pat dry; if desired, cover and chill up to 1 day.
In a small bowl, mix 1 1/2 teaspoons salt, pepper, and horseradish. Set beef on a rack in a 9- by 13-inch pan. Pat horseradish mixture over the top and sides of beef.
Roast in a 375° regular or 350° convection oven until a meat thermometer inserted in center of thickest part reaches 120° to 125°, 1 1/2 to 2 hours. Remove from oven and let stand in a warm place at least 20 minutes.
Transfer meat to a rimmed platter or board; scrape any pan juices into a small bowl. Cut meat across the grain into very thin slices (see notes). Spoon pan juices onto portions.