Cook broth with lavrushka and pepper from fish. Remove the finished fish, set aside.
Cut the potatoes for the German fish soup into small cubes, pour into the broth, cook until tender. Cut the onion into small cubes, sauté in butter.
Free cooled fish from skin and bones, cut. Pour cream into the soup, add sautéed onions, herbs, fish slices, and salt.
Bring the soup almost to a boil and immediately remove from heat.