How to Make It
In a small saucepan, combine the mirin and sake and boil for 1 to 2 minutes. Add the miso and sugar and simmer, stirring to dissolve the sugar, until slightly thickened. Transfer to a small bowl and let cool completely. Serve drizzled over sliced avocado.
The sauce can be used as a marinade for tuna steaks; cover the fish with it and refrigerate for 1 hour before cooking to your liking.