Peel and finely chop the onion. Clean the mushrooms, rub them with kitchen paper if necessary and cut into slices. Heat oil in a large pot. Fry the mushrooms briefly, take them out and put them aside.
Sauté onion in hot frying fat for about 5 minutes. Deglaze with 1 1⁄2 l water. Add mixed mushrooms, bay leaf, juniper and peppercorns. Bring to the boil and simmer for about 10 minutes.
Peel and wash the carrots and celery. Clean and wash the leek. For the soup 1 carrot, approx. 100 g celery and 1⁄2 finely dice the leek. Cut the rest of the vegetables coarsely, add them to the mushroom stock and simmer for about 30 minutes.
Pour broth through a sieve into a pot. Season to taste with salt and sherry. Wash parsley, shake dry, chop leaves. Add vegetable cubes and mushrooms to the broth about 5 minutes before serving, simmer again for another 5 minutes.
Pour soup into cups and sprinkle with parsley.