Having left the tuna in the fridge sufficiently long to gradually defrost through and through (around 12 hours), dab dry with kitchen towel and leave to acclimatise to room temperature.
Peel the potatoes and cook in salted water.
Heat the milk with the butter. Once hot, add to the cooked potatoes and beat into a creamy mash. Gradually work in the wasabi paste.
Roughly chop the herbs.
Wash the lemons thoroughly, dry them and finely grate the rind.
Season the tuna steaks with salt and pepper, drizzle with olive oil and coat with the lemon rind.
Heat a pan as you would for beef steak. Place the rind-coated fish in the pan and turn every 20 seconds or so to seal the tuna on all sides. This done, leave the fish to rest.
Spoon out the mash into the centre of a shallow dish, sprinkle the herbs on top of the mash and then gently place the tuna on top. You may choose to squeeze a few drops of lemon juice on top.
Tuna ought always to be served pink in the middle.