Pat steak dry and sprinkle all over with 1/2 teaspoon salt and 1/2 teaspoon pepper.
Broil steak on rack of broiler pan 3 inches from heat 4 to 5 minutes per side for medium-rare. Transfer to a cutting board and let stand, loosely covered with foil, 5 minutes.
Stir together sour cream, horseradish, and remaining 1/4 teaspoon salt and 1/4 teaspoon pepper in a small bowl and serve with steak.
