1. Whisk soy sauce, rice wine, sesame oil and garlic in a large bowl. Add flank steak and toss to coat. Let stand 10 minutes.
2. In large nonstick skillet or wok over high heat, warm oil until hot. Add flank steak and cook, stirring often, until it loses its pink color, 2 to 3 minutes. Stir in scallions and sesame seeds, stir-fry for 1 minute longer and serve.