Stir together soy sauce, veinegar, ginger, garlic, Sriracha, sugar and sesame oil.
Prepare a gas grill for direct-heat cooking, and grill the steak, covered, turning over once for 6-8 minutes for medium-rare.
Transfer steak to a cutting board and drizzle with 2 tablespoons sauce, then let stand, uncovered, 5 minutes. Thinly slice steak across the grain. Serve with remaining sauce, scallions, and sesame seeds.