Soak the dried mushrooms in the 2 litres of hot water for half an hour.
Cook the pearl barley in salted boiling water for 20 - 30 minutes until soft. If using rice or buckwheat these will cook more quickly. Drain and set aside.
Drain the mushrooms, leaving behind any sandy residue. Reserve the liquid and pour it into a soup pot. Chop the mushrooms finely and add to the pot along with the whole onion. Season lightly. Cook for 10 minutes.
Add the potatoes and cook for 15 minutes.
In the meantime, slice half the gherkins and grate the other half.
Heat butter and oil in a large pan over a medium heat and add the diced onion. Cook until transparent and softened. Add the carrot, parsnip and celeriac and cook for 5 minutes. Add some water if pan is too dry. Now add the garlic and cook for a couple of minutes. Then add all the gherkins and the brine. Simmer for a minute.
Add the contents of the frying pan to the mushroom broth in the soup pot along with the cooked pearl barley. Bring to the boil. Remove the whole onion.
Serve with marjoram leaves (optional) and rye bread spread with creme fraiche.