In a small pan heat together the rice wine, mirin, japanese soy sauce and sugar, stirring until the sugar has melted. Set aside.
Preheat two griddle pans. Brush the tuna steaks with sunflower oil and cook on the griddle pans until just cooked to taste, carefully turning over to brown both sides.
Spoon over the sauce mixture and cook again until glazed, again turning to glaze both sides. Serve immediately.