Heat a 2½-quart nonreactive heavy-bottomed saucepan over medium heat, and when hot, add the peanut oil. When the oil is hot, stir in the scallions, ginger, shallot, garlic, and dehydrated onions. Cook the vegetables, stirring occasionally, until softened and very fragrant, about 3 minutes. Stir in the soy sauce, water, mirin, sherry, and honey and bring the liquid to a brisk bubble through the center. Turn the heat down to low and simmer the sauce for 10 minutes with the cover ajar.

Meanwhile, mix the cornstarch and vinegar in a small bowl. After the sauce has simmered for 10 minutes, stir in the dissolved cornstarch. Bring it back to a bubble, then let it simmer, uncovered, for 5 minutes. Stir in the toasted sesame oil and the wasabi paste, if using. Remove from the heat and let the flavors meld until completely cool. Strain the cooled sauce through a fine-mesh sieve that’s positioned over another bowl, pressing hard on the solids. Store in the refrigerator in a clean, securely sealed jar.