Combine the dressing ingredients in a small bowl and set aside.
Bring 2 quarts of lightly salted water to a boil in a large pot over high heat. Separate the noodles and drop them into the boiling water, stirring once or twice. When the water begins to boil, add 1 cup of cold water. Repeat this procedure twice cooking until the noodles are just tender, about 3 to 4 minutes. Drain in a colander. Rinse with cold water until completely cooled, rubbing gently with hands to remove all surface starch and drain well.
Toss the noodles with the dressing and arrange the daikon, sprouts, onion, green onions, sesame seeds, and a dash of seven-spice powder over the top.