1. Preheat broiler.
2. Whisk together buttermilk, mayonnaise, zest, horseradish and scallion. Allow to sit for 15 minutes while preparing and cooking steak.
3. Season steak with salt and pepper. Broil 4 inches from heat until desired degree of doneness, about 4 to 6 minutes per side. Allow to sit for 5 minutes before slicing thin against the grain.
4. Divide greens amongst 4 dinner platters. Top each with about 3 ounces of steak and drizzle each with about 2 1/2 tablespoons dressing.