You'll need only 1/4 cup of this for the New-Style Sashimi Salad, but it keeps up to four days in the refrigerator. It is delicious on grilled meats.
Pulse onion in a food processor until finely chopped. Transfer to a fine sieve set over a small bowl and let drain.
Place 3 tablespoons drained onion, soy sauce, vinegar, water, sugar, salt, mustard, and pepper in a bowl and whisk until sugar and salt are dissolved and mixture is combined. Whisk in oils until combined.
Adapted from Nobu: The Cookbook, by Nobuyuki Matsuhisa (Kodansha U.S.A., 2001)