Combine the dashi, soy sauce, and mirin in a small saucepan. Place over medium heat. As soon as the dipping sauce comes to a boil, turn off the heat. When you're ready to serve, reheat over low heat.

Grate the daikon on the coarsest side of a box grater. Squeeze out excess liquid and set aside. Grate the ginger finely. Serve the grated daikon and ginger on the side of the dipping sauce. When you're ready to eat, add the daikon and ginger to the dipping sauce and dip the tempura into it.