Prepare sauce: Mix the mustard powder and hot water to form a paste. Let sit for a few minutes. Add vinegar and soy sauce mix together, and strain. Chill.
Warm the Buerre Blanc in a double boiler. Mix all the spices together on a plate and dredge the ahi on all sides. Heat a lightly oiled cast-iron skillet and sear the ahi over high heat to desired doneness (about 15 seconds per side for rare). Cut into 16 thin slices.
For each serving, arrange 4 slices of the ahi in a pinwheel. Ladle a little of the Soy-Mustard Sauce in two opposing quadrants, ladle the Buerre Blanc in the other two quadrants of the plate. Garnish decoratively with the pickled ginger on the Buerre Blanc and the sesame seeds on the Soy-Mustard Sauce. Arrange remaining garnish at the center of the pinwheel.