Buckwheat noodles, called soba, a Japanese stable, are the backbone of this classic composed salad. It also includes shiso leaves, daikon, sake, and radish sprouts -- plus a flurry of deep-fried tempura crumbs for crunch.
Make the dressing: Cook soy sauce, sake, mirin, and 2 tablespoons water in a medium saucepan, stirring occasionally, over medium heat until bubbling, about 4 minutes. Add bonito flakes. Remove from heat, and let cool completely. Pour through a fine sieve into a small bowl; discard bonito flakes.
Bring a large pot of water to a boil. Add noodles; cook according to package instructions, about 5 minutes. Drain, and rinse under cold running water.
Arrange noodles in 4 serving bowls; drizzle each with 1 tablespoon dressing. Arrange nori, radish, shiso leaves, radish sprouts, and tempura crumbs around bowl. Serve with remaining dressing.