Combine koikuchi soy sauce, mirin, and sake in a small saucepan. Bring the mixture to a gentle simmer over medium heat, then immediately remove from heat.
2
Add the kombu to the warm marinade. Let the marinade cool completely to room temperature.
3
Pat the sashimi-grade tuna dry with paper towels. Slice the tuna against the grain into 1 cm thick pieces.
