In a mixing bowl, combine the first 10 ingredients together. Mix well. In a small bowl, whisk the mustard with 1 1/2 tablespoons warm water until a paste forms. Gradually whisk in the soy sauce. Set aside. In a saucepan, boil the wine, vinegar, shallot and peppercorns, until almost no liquid remains in the pan, about 2 minutes. Add the cream and boil until slightly thickened, about 1 minute. Season with salt and pepper. Set the sauce aside. In a mixing bowl, whisk the flour, water and egg together. Season with salt and pepper. Set aside. Season the ahi with salt. Spread the wasabi evenly over 4 nori half-sheets. Place 1 piece of the ahi one end of each nori sheet. Fold the nori over the ahi to cover (the ends will be exposed). In a medium-saucepan, heat the oil. Dip 1 ahi roll in the soy sauce and then in the flour; shaking off any excess flour. Dip the rolls in the tempura batter and fry in the oil until the outside is crispy and the tuna is rare inside, about 40 seconds. Remove the ahi from the oil and drain on a paper-lined plate. Season with salt and pepper. Repeat the remaining rolls. Bring the reduction sauce to a simmer. Whisk in enough mustard to desired taste. Whisk in the butter. To assemble, spoon the sauce in the center of the plate. Pile the relish in the center of the sauce. Slice the tempura ahi and lay them directly on top of the relish. Garnish with parsley.
Adapted from the New Cuisine of Hawaii Cookbook