Preheat a grill or a broiler.
Sprinkle a little sea salt and pepper on the tuna. Briefly sear the tuna on all sides until its surface turns white. Plunge the fish into a bowl of ice water to stop it cooking any further. Shake off the excess water. Cut the tuna into 1/8-inch thick slices. Toss the greens with some of the Matsuhisa dressing and place on a plate. Top with the sliced tuna. Drizzle with a little more dressing.
Combine all of the ingredients except the oils. When the salt is fully dissolved, add the oils. Set aside.