2 tablespoons prepared Chinese mustard (I use Beaver brand "extra-hot")
2 tablespoons ponzu sauce (a citrus-seasoned soy sauce; Kikkoman makes one) or regular soy sauce
1/2 cup dry white wine
1 tablespoon rice vinegar or white-wine vinegar
1 1/2 tablespoons minced shallot
Pinch ground white pepper
3 tablespoons whipping cream
1/2 cup chilled unsalted butter, cut into 32 pieces (cut cubes lengthwise into quarters, then cut crosswise into 8 pieces, which will produce 32 chunks!)
In a small bowl, whisk together the mustard and the ponzu or soy sauce; set aside (you'll add it to butter sauce at the very end of cooking).
In a small pot, combine the wine, vinegar, shallot and white pepper. Bring to a boil and simmer until it is reduced to about 4 tablespoons (this will only take about 5 or 6 minutes). Whisk in the cream and boil just until it begins to thicken and reduce slightly, about 1 minute. Set reduction aside until just before serving.
Sauce may be prepared up to this point several hours ahead. Don't add the butter until just before serving.
When ready to finish sauce, bring reduction to a boil. Turn heat to low, then whisk in chilled pieces of butter 1 or 2 at a time. Keep whisking steadily until all butter has been incorporated. Keep sauce over very low heat (or in top of a double boiler set over hot water) or sauce will separate. Whisk in about half of reserved mustard-ponzu mixture. Taste and then add more mustard mixture as desired.
Makes about 1 1/3 cups sauce; enough for 2 to 3 pounds of grilled Pacific albacore (which will feed 6 to 8 people).