Make a teriyaki sauce by mixing the soy sauce, mirin and sugar in a shallow dish. Put the tuna steaks in the dish, turn over a few times so they’re coated with the sauce, then leave in the fridge to marinate for at least 1 hr and up to 2 hrs.
To cook, heat a large frying pan. Take the tuna steaks out of the marinade and pat dry, then rub all over with the oil. Fry the tuna for 2 mins on each side, so they are still pink in the middle. Remove from the pan, then pour in the marinade and 2-3 tbsp water. Bring to the boil and cook for a few mins to heat through. Strain through a sieve into a small dipping bowl.
To serve, slice the cooked tuna, mango and spring onion into bite-size pieces. Thread onto wooden skewers, then serve warm or cold with the dipping sauce.