Mix the marinade ingredients together, stirring to dissolve the sugar.
Bring a large pot of water to a boil. Add salt and the asparagus.
Cook until bright green and tender but still firm, just a few minutes. Scoop the asparagus out, rinse it under cold water, and set on a towel to dry.
Pull the noodles apart with your fingers, add them to the boiling water, and give them a quick stir. Boil until tender but not overly soft, tasting them often as they cook. It should take only a few minutes. Pour the noodles into a colander and immediately rinse under cold water. Shake off the excess water.
Toss the noodles with all the marinade and most of the scallions, sesame seeds and asparagus.
Mound them in a bowl or on a platter, then garnish with the remaining asparagus, scallions and sesame seeds.