Prepare the mushrooms: wash, boil in salted water until tender (15 minutes after boiling).
While the mushrooms are cooking, marinate the onion. To do this, cut the onion thinly into quarters, put it in a deep bowl, sprinkle with sugar (about a tablespoon without a slide), salt (a teaspoon without a slide), mix well and pour vinegar (3 tablespoons). Leave it on for 15 minutes.
Cool the boiled mushrooms and cut into thin slices.
Cut the seaweed so that it is not a long straw (otherwise the products for mixing will be distributed unevenly).
Add boiled mushrooms, onions, finely chopped walnuts to the seaweed. Mix everything thoroughly.