Place karashina on a heatproof plate lined with plastic wrap, cover, and microwave at 600W for 2 minutes. Rinse under cold water and drain.

Put karashina and salt into a storage bag, massage by hand, and let sit for 10 minutes. Squeeze out excess water, cut off the roots, separate leaves and stems, and finely chop the stems.

In a bowl, mix cooked rice, chopped karashina, salted kombu, and white sesame seeds. Divide into 6 portions.

Lay two karashina leaves on plastic wrap, place one portion of rice mixture on top, and wrap the leaves around the rice to form a neat shape.