Soak the dried mushrooms in 1 cup hot water for 15 minutes. Drain, reserving the liquid, and squeeze out the excess liquid. Strain all the soaking liquid through a fine-mesh sieve. Reserve the liquid (there should be 1 cup) and the mushrooms separately.
In a tall narrow stockpot, heat the oil over medium heat. Stir in the fresh mushrooms and cook, stirring occasionally, for 4 minutes. Stir in the onion and cook, stirring occasionally, for 8 minutes.
Add the beets, carrots, parsnip, celery root, 8 cups water, and the mushroom soaking liquid. bring to a boil. Lower the heat and simmer for 5 minutes. Stir in the potatoes, cabbage, garlic, and, if using, the beet greens. Dissolve the tomato paste in 1/2 cup of the liquid and stir back into soup. Return to a boil. Lower the heat and simmer for 5 minutes. Stir in the reconstituted mushrooms and simmer for 5 minutes, or until all the vegetables are tender.
Remove from the heat. Stir in the dill, sugar, vinegar, salt and pepper.
Pass bowls of chopped dill and sour cream at the table.
Courtesy of Soup, A Way of Life, by Barbara Kafka