Cut fish into serving pieces
Put fish in a plastic bag and marinate for 12 hours in Saikyo Miso (see recipe below), turning every now and then.
Drain fish, reserving 3 Tablespoons of the marinade for the Dashi sauce (recipe below).
Broil fish for 2 to 3 minutes to caramelize the flesh. Turn, skin side up, and broil for another 2 to 3 minutes. Serve with Dashi sauce.
Simmer the ingredients gently for about tem minutes and taste. You are looking for just the right balance (to your taste) of salt, sweet and tang from the yuzu. To adjust the sweet add some of the reserved Saikyo Miso or a bit of sugar, for salt, add salt. Be careful with the yuzu as the flavor can be very dominating. Its better to start with less and bring it up gradually to get it where you want it to be.
Combine miso and sugar in the top of a double boiler. Add sake and mirin, and whisk to combine, using a heavy whisk.
Cook over boiling water, stirring frequently until sugar is dissolved and the color begins to darken, 30 to 45 minutes. Remove from heat, and cool.