Combine the mirin and sake in a small saucepan over medium high heat. Bring to a simmer then add the soy sauce and bring to a boil.
Boil for about a minute then kill the heat. Pour the marinade into a heat proof bowl and chill it before adding the fish. The fastest way to chill the marinade is submerging the container in an ice bath. Prepared marinade can be stored in an airtight container in the fridge for up to 3 weeks.
Place the sliced fish into the chilled marinade. Marinate for 10 minutes than remove and pat dry with paper towels. Discard used marinade.
Notes
If this is your first time making sushi be sure to check out the Sushi Beginner's Guide before getting started!