Core and slice tomatoes in half horizontally. Remove seeds.
Arrange tomato halves, cut side down, on paper towels and let drain 30 minutes.
Preheat grill to medium-high.
Lightly coat grill grate with cooking spray. Place tomato halves, cut side down, on grate and cook 5 minutes. Turn tomatoes and grill 1 additional minute or until skin is charred. Remove tomatoes from heat and let stand 5 minutes.
Chop tomatoes into 1" pieces.
In small mixing bowl, combine tomatoes, onion, vinegar, oil, 1/8 teaspoon salt, 1/8 teaspoon pepper, and basil.
Season both sides of steak with remaining salt and pepper.
Place steak on grill and cook 6 minutes per side or until internal temperature reaches 145°F.
Remove from heat and let stand 5 minutes.
Slice steak diagonally into thin pieces. Serve with tomato salsa on side or over steak.