Pour 3 tbsp mirin and 3 tbsp sake into a small saucepan and bring to a boil over a medium-high heat. Boil for 1 minute and then turn off.
Add 3 tbsp Japanese soy sauce (koikuchi shoyu) and swirl around. Transfer to a wide container and allow to cool to room temperature. (You can speed up the process by resting the container of the marinade over a bowl of ice or cold water.)
Once the marinade is cool to the touch, place 3 g dried kelp (kombu) in it.
Dip 150-200 g sashimi-grade tuna pieces to coat both sides and then lay them flat in 1 layer without overlapping.
Cover with the plastic wrap making direct contact with the tuna and marinate in the refrigerator for 30 mins.
Once 30 mins have passed, lift the kombu out of the marinade. Dish up the warm rice and arrange the marinated tuna pieces on top. Pour ½ tbsp of the leftover marinade over each bowl and garnish with sesame seeds, an egg yolk, shredded perilla leaves and a blob of wasabi (optional).
Enjoy!
Notes
Marinating past 30 minutes: Go ahead. The longer it sits, the darker the tuna turns and the stronger it tastes, with the salt of the soy sauce coming forward and the tuna's own flavor stepping back. Stop whenever it tastes right to you.Cool the marinade all the way first: Tuna that goes into still-warm marinade turns white and firm wherever the liquid touches it, and those spots lose their raw sashimi texture.Press the wrap onto the fish: There is not enough marinade to cover the tuna. Wrap pressed against the fish holds the coating on both sides in place. Wherever the wrap lifts, the top marinates more lightly.The marinade won't make fish safe to eat raw: Soy sauce and wasabi do not kill parasites in fish. Use only tuna labeled for sashimi or for eating raw.Other raw seafood: Salmon, sea bream, scallops or any other seafood labeled for eating raw takes the same marinade for the same 30 minutes.Eat it the same day: Serve the marinated tuna the day you make it. Marinating doesn't extend the date on the package.