To make Sauce: In saucepan, mix 3 cups water, soy sauce, sugars, garlic, ginger and scallions; bring to boil. Mix 2 tablespoons water with cornstarch. Gradually mix into soy mixture over low heat. Cook and stir until sauce coats back of spoon; keep warm.
For each serving, to order: Heat 2 tablespoons oil in small skillet until almost smoking. Cook 1 salmon fillet until golden brown and cooked throughout, turning once. Nap plate with 3 tablespoons Sauce. Plate salmon on sauce, skin side down.