Preheat a large grill pan over medium heat. Sprinkle steak on both sides with 3/4 tsp. salt and 1/4 tsp. pepper. Make sure pan is hot, then cook steak for about 6 minutes per side, turning once, for medium-rare. Transfer to a cutting board, cover loosely with foil and let stand for 5 minutes.
Combine watercress, tomatoes and radishes in a large bowl. Drizzle oil and vinegar on top; toss well. Season with remaining 1/4 tsp. each of salt and pepper, then toss again.
Thinly slice steak and divide among 4 dinner plates. Top each portion with salad. Sprinkle crumbled blue cheese on top, if desired.