In a small saucepan, heat the rice wine vinegar and mirin on medium. Bring to a simmer, then turn off the heat.
Tear the nori sheet into small pieces and then add them to the saucepan as well as the shiitakes.
Pour in the citrus juice and soy sauce. Stir to combine everything.
Let cool completely, then pour into a container with a lid. Cover and put in the fridge.
Let chill for at least 4 hours, but I like to leave it overnight.
When ready, strain the ponzu sauce into a bowl and discard the nori and shiitakes.
Return the sauce to a container with a lid and keep in the fridge until ready to use.
Keeps for a few weeks in the fridge. Use for dipping veggies, sushi or anything you want. Add to sauce or steamed veggies.