Cook the sushi rice according to the package.
Mix the vinegar, sugar and salt and slowly heat while stirring in a sauce pan until the salt and sugar are dissolved.
Once both the sushi rice and vinegar mixture are at room temperature mix both together.
Rinse the fermented mushrooms to remove excess salt and squeeze out as much liquid as possible.
Chop the mushrooms into small pieces.
Combine the chopped mushrooms, mayonnaise and chili flakes and mix well.
Put a nori sheet on the sushi roller and with wet hands make a layer of rice on the nori sheet covering about 2/3 of the sheets surface.
Add some of the spicy mushroom mixture in the center.
Tightly roll up the sushi roll until it is closed. Use some water or rice grains to make the ends of the sheet stick together. Repeat the same procedure for the remaining nori sheets.
Wet the blade of a sharp knife to make sure the knife will not stick much to the sushi rice. The easiest way to do is is to have a tall container filled with water in which you can submerge the blade of the knife. Cut the sushi rolls into 8 pieces. Regularly wet the blade of the knife in between cutting.