Bonito flakes, mirin and light soy sauce are available at Japanese groceries and in the ethnic sections of many grocery stores. Add one-half cup of grated daikon radish, if desired, to the sauce before serving.
1 cup bonito flakes
1 cup water
1/4 cup mirin
1/4 cup light soy sauce (usukuchi shoyu)
In a medium saucepan, combine the bonito flakes, water, mirin and light soy sauce and bring to a boil over high heat. Remove from the heat and strain the broth through a cheesecloth or paper towel-lined sieve into a bowl, discarding the bonito flakes. This makes about 1 cup sauce. The sauce can be made 1 day ahead and refrigerated; before serving the tempura, reheat the dipping sauce and divide it among 4 bowls.