Make the tare: combine the sake, mirin, sugar, soy sauce, and dashi powder in a small saucepan. Bring to a boil, then cool to room temperature.

Chop the sashimi-grade tuna into small pieces, then mince finely by repeatedly dropping the knife blade onto the pile until you get a slightly chunky paste.

Place the warm rice in a serving bowl and level the surface. Position a perilla leaf on one side of the rice if using.

Spread the minced tuna evenly over the rice and perilla leaf.

Scatter the chopped green onions over the tuna and pile kizami nori in the center. Place a dab of wasabi on the side.

Pour the cooled tare evenly over the tuna and serve immediately.