Prepare the mold by cleaning and drying a 500ml milk carton. Close the top, tape it shut, and cut one side open. Line the inside with plastic wrap, leaving extra wrap hanging over the edges. Prepare three molds this way.

Rinse the rice and drain. Add it to a rice cooker with 360ml water and cook as usual (use sushi rice mode if available).

Mix the vinegar mixture ingredients in a bowl and let it sit for 30 minutes to 6 hours to blend the flavors.

Blanch the nanohana in salted boiling water, then cool in ice water. Pat dry. Wipe the kombu with a sake-soaked paper towel. Layer kombu, sea bream, kombu, salmon, kombu, nanohana, and kombu in a container. Sprinkle a pinch of salt between layers. Cover and refrigerate for at least 1 hour.

Transfer the hot rice to a large bowl. Pour the vinegar mixture over the rice and mix gently using a cutting motion to avoid breaking the grains. Fan the rice to cool and bring out its shine. Cover with a damp cloth and let it cool to body temperature.

Divide the sushi rice into 6 portions. In one mold, layer sea bream slices, one portion of rice, a sheet of nori, another portion of rice, and press firmly. Wrap with the plastic wrap. Repeat with salmon and nanohana in separate molds. Refrigerate for 30 minutes.