Combine vinegar, soy sauce, ginger and garlic in a high-top skillet with a lid. Pat the fish dry and season both sides with salt and pepper. Place fish in the pan and bring to a simmer over medium-low heat. Cover and steam for about 5 minutes, until fish turns opaque.
Meanwhile, in a separate small saucepan, heat the sesame oil. Add scallions and sesame seeds and cook until scallions begin to turn golden. Remove from heat. Plate fish; pour remaining liquid on top and drizzle with scallion sesame mixture.