Preparation
(the sauce)
1. In a small bowl, mix the wasabi powder with the water. Make sure to mix thoroughly, to form a thin paste.
2. In a larger bowl, mix the remaining sauce ingredients, then add the wasabi/water paste
3. When everything's incorporated, cover the bowl with plastic wrap, and refrigerate. Allow 30 minutes or so for the flavors to blend.
4. Preheat grill or broiler.
In a small bowl, combine all tuna dressing ingredients (except for the tuna itself).
5. Brush the prepared dressing liberally onto both sides of each tuna steak.
6.Put the steaks onto a hot grill (or under a preheated broiler), and allow them to took on one side for at least 3 minutes.
7. After 3 minutes, flip the steaks and cook for another 2 minutes. Preferably, the tuna should be somewhat rare in the center.
8. Plate the tuna, and drizzle liberally with the wasabi mayonnaise.