Sprinkle the steak on both sides with salt and pepper. Brush the grill grates with oil and carefully place the steak on the hottest part of the grill. Cook until the internal temperature is 135 to 140 degrees F, about 8 minutes per side. Remove the steak and let it rest on a cutting board for about 10 minutes.
Meanwhile, mix the sour cream, mayonnaise, horseradish, Worcestershire and 1 tablespoon parsley in a small bowl. Slice the steak 1/2-inch thick. Spread half of the horseradish sauce on a platter, then arrange the steak on top of the sauce (keep the bone on the platter to show it is a porterhouse). Drizzle a little more sauce on and sprinkle with the remaining parsley.