Place tomato halves, cut sides down, on paper towels. Let stand 30 minutes.
Heat a grill pan coated with cooking spray over medium-high heat. Place tomato halves, cut sides down, in pan; grill 5 minutes. Turn tomato halves; grill 1 minute or until the skin is blackened. Remove from pan; cool 5 minutes. Cut tomato halves into 1-inch pieces. Combine tomato, onion, vinegar, oil, 1/8 teaspoon salt, 1/8 teaspoon pepper, and basil.
Sprinkle steak with 1/8 teaspoon salt and 1/8 teaspoon pepper. Place steak in pan coated with cooking spray; grill 6 minutes on each side or until desired degree of doneness. Let stand 5 minutes. Cut the steak diagonally across grain into thin slices. Serve with the tomato salsa.