How to Make It
In a medium saucepan, mix the dashi with the mirin and soy sauce. Whisk in the diluted cornstarch and simmer over low heat until reduced to 1/2 cup, about 30 minutes.
Season the cod with salt. Top each fillet with 2 tablespoons of the grated daikon. Place the fillets in a steamer set over simmering water and steam over moderate heat until the fish is just cooked through, about 5 minutes.
Transfer the cod to plates. Spoon the sauce around the fillets, garnish with lemon zest and serve.