Warm over low heat to cook off the alcohol (this is called nikiri).
Let cool.
Steeping and Straining
Put the citrus juice, the cooled mirin and sake, the soy sauce, the katsuobushi and the kombu in a clean glass jar or bottle.
100 ml Japanese soy sauce,, 10 g katsuobushi, 1 piece kombu
Mix well.
Seal the container.
Let sit overnight in the fridge.
Strain the sauce through a fine-mesh sieve or a tea strainer.
Remove the katsuobushi and kombu.
Notes
The base of this sauce is simple: 1 part soy sauce to 1 part citrus juice. From there, you adjust it to taste with a little dashi, mirin or rice vinegar. Freshly made, the sauce has a very sharp citrus flavor; a little resting time rounds it out.Fresh yuzu is rare and expensive in Europe. The best substitute is bottled 100% yuzu juice, sold at Asian grocery stores or online; use it in the same amount (100 ml). Without yuzu, two blends work well for 100 ml of juice: 50 ml lemon and 50 ml lime; or 50 ml lemon, 30 ml lime and 20 ml grapefruit, which adds yuzu's slight bitterness. Lime juice quickly turns bitter once squeezed: squeeze the citrus just before making the sauce.Nikiri means heating the mirin and sake to drive off the alcohol: without this step, the sauce keeps a harsh alcoholic taste.Use a glass or ceramic container, scalded with boiling water before use: the sauce is acidic and takes on a metallic taste in a metal container.Once strained, the sauce keeps in the refrigerator in a clean glass bottle. This version, made with fresh juice and no vinegar, keeps for about a month. It can also be frozen in an ice cube tray.