Put the stock, soy sauce, sake, vinegar, sugar and 1tbsp of the oil into a saucepan. Bring to the boil and cook uncovered for 15 minutes or reduce by about one third.
Brush the tuna steaks with the remaining oil and coat the edge of the tuna with the sesame seeds. Cook the noodles in the soup according to packet instructions. Heat a ridged frying pan until very hot and cook the tuna for 2 to 3 minutes each side.
Transfer the noodles to serving bowls, pour over the stock, top with the tuna steaks and garnish with coriander. Serve immediately.