This is a classic appetizer in Japan, where it is served from late winter through early spring and again in the autumn. On warm days it’s served with a frosty glass of cold beer; on chilly days it’s best with warmed saké.
6-8 square inchesdashi kombu (kelp for stock making)
Soak the mussels in salted water to cover for 20-30 minutes to help disgorge any sand. Scrub the mussels and remove their beards.
Place the mussels in a wide-mouthed pot. Add the kelp and wine, cover, and cook over high heat until the mussels open, about 3 minutes. Discard any unopened mussels after 4 or 5 minutes. Strain the broth through a cloth- or paper-lined strainer and reserve it. Remove the mussel meats from their shells and let them cool in the reserved broth while you make the mustard sauce.
Combine the bean paste, sugar, and 1tablespoon of the reserved mussel broth in a small saucepan. Stir to combine well. Cook the sauce over medium heat, stirring, until bubbly and glossy, about 2 minutes. Remove the sauce from the heat. Combine the vinegar and mustard powder to make a paste, and stir this into the sauce.
Trim the scallions and cut them into ¾-inch lengths. In a small saucepan, blanch them in boiling water to cover for 20 seconds, just long enough to wilt them yet keep the color vivid. Drain and rinse under cold water. Gently press out all excess liquid from the scallions.
The sauce, mussels, and scallions can all be chilled separately at this point if you wish to serve this dish more than 30 minutes later. Just before serving, remove the mussel meats from their broth and combine them with the blanched scallions. If the mustard sauce seems very thick (like tomato paste rather than yogurt), or if you want to intensify the mussel flavor, thin the sauce slightly with a few drops of mussel broth. Toss the mussels and scallions in the mustard sauce to mix well. Serve them warm or at room temperature in small individual mounds.