In a saucepan, combine the soy sauce, sake, brown sugar and mirin. Let boil, then add cornstarch mixture, stir and cook until thick. Set aside.
Scale the salmon and then wash under cold running water. Pat dry with paper towels. Season with salt and pepper and then coat lightly with flour.
Heat oil in a non-stick frying pan over medium high heat. Add salmon belly, skin side down. Fry until skin is golden brown, about 3 minutes. Flip the salmon belly over and fry the other side for 2 minutes or until a golden crust is formed. Drain on paper towels.
Pour off all but 1 tablespoon of oil from the pan. Add onions and carrots and sauté for 3 minutes. Add cabbage, bean sprouts and 3 tablespoons of the teriyaki sauce. Stir-fry for a minute.
To serve, place vegetables over rice. Top with salmon belly. Pour sauce over the salmon belly and sprinkle with sesame seeds and green onions.