Mix together the soy sauce, vinegar and mirin. Pour over the tuna and marinate for at least 20 mins or up to 2 hrs in the fridge. Place the potatoes in a pan of lightly salted boiling water, then cook for 10-15 mins until soft. Drain well. Heat the milk in the pan and mix in the wasabi, return the potatoes to the pan, then mash until smooth. Stir through the spring onion and keep warm.
Heat a non-stick griddle pan until smoking hot. Remove the tuna from the marinade. Cook on the griddle for 2-3 mins on each side until seared on the outside, but still pink inside.