In a large sauté pan, heat the sesame oil until smoking. Season the tuna with the salt and pepper, and sear each side in the hot oil. You should cook the tuna just long enough to achieve a subtle brown color on each side. The tuna should not be cooked to more than medium rare.
Cool the tuna immediately and prepare the sauces.
Mix together all ingredients and chill before serving.
In a small mixing bowl, blend together the Wasabi powder and hot water to form a thick paste. Add in the remaining ingredients and blend well.
To serve the tuna:
Use a very sharp knife to cut very thin cross-section pieces of the tuna (1/4-inch discs). Arrange the slice tuna on a serving platter (as you would when spreading a deck of cards) and drizzle with the two sauces.
Note: When drizzling the sauces, do not cover the bright red tuna completely. The presentation is best when the bright red of the tuna is visible.