1. Put potatoes in a saucepan and cover with cold water by 2 inches. Stir in 1 tsp. salt. Cover and bring to a boil over high heat. Reduce heat to medium-low and simmer until potatoes are tender when pierced with a fork, 12 to 15 minutes. Drain. Slice potatoes in half lengthwise.
2. In a medium bowl, whisk together buttermilk, chives, mayonnaise, sour cream, lemon juice, Worcestershire sauce and garlic.
3. Preheat a grill to high. Pat steak dry and season with pepper and remaining 1/4 tsp. salt. Oil grill grates. Grill steak, turning once, about 6 minutes total for medium. Transfer steak to a cutting board, cover loosely with foil, and let rest 10 minutes. Slice steak against the grain.
4. Arrange watercress, radishes, potatoes and steak on 4 serving plates. Drizzle salads with dressing, crumble blue cheese on top and serve.