1Prepare grill for cooking over direct heat with medium-hot charcoal (moderate heat for gas).
2While coals are lighting, let steaks stand at room temperature about 15 minutes. Stir together sour cream, horseradish, scallions, 1/2 teaspoon salt, and 1/2 teaspoon pepper in a bowl.
3Pat steaks dry and sprinkle all over with remaining teaspoon each of salt and pepper. Grill steaks on lightly oiled grill rack, covered only if using a gas grill, turning steaks over once and moving them around grill to avoid flare-ups if necessary, until thermometer inserted horizontally 2 inches into meat (do not touch bone) registers about 120°F for rare, 9 to 12 minutes total. Transfer steaks to a cutting board and let stand 10 minutes before slicing. (Internal temperature will rise to 125 to 128°F while steaks stand.) Thinly slice meat and serve with sauce.
If you aren't able to grill outdoors, steaks can be seared in a hot lightly oiled well-seasoned large (2-burner) ridged grill pan over moderately high heat, turning over once, until grill marks form, 4 to 6 minutes total. Reduce heat to moderately low, then cover with 2 large overturned metal bowls and cook, turning over once, about 12 minutes total.