Cut the tuna into thin, long strips. Mix mirin and soy sauce in a tray, add the tuna, and mix well. Cover with plastic wrap and marinate in the fridge for 10 minutes.

Mix cooked rice with sushi vinegar in a bowl to make sushi rice.

Cut takuan into thin strips and trim the stems off the shiso leaves.

Place two sheets of nori vertically on plastic wrap, overlapping slightly. Use a small amount of sushi rice to connect the sheets into one large piece. Repeat to make two large nori sheets.

Spread half of the sushi rice on one nori sheet. Arrange shiso leaves, tuna, and takuan at the edge closest to you, then roll tightly from the edge.

Spread the remaining sushi rice on the second nori sheet, sprinkle white sesame seeds, and place the first roll on top. Roll tightly again, wrap in plastic wrap, and let sit for 10 minutes.