Preheat oven to 400 degrees. Place beets on a piece of parchment paper -- lined aluminum foil. Sprinkle with salt and pepper. Fold up edges of foil and parchment to enclose beets. Place on a small baking sheet, and bake until tender when pierced with the tip of a knife, about 1 hour. Let stand until cool enough to handle. Peel, and coarsely grate; set aside.

Soak mushrooms in hot water for 30 minutes. Drain through a fine-mesh sieve, reserving liquid. Coarsely chop mushrooms; set aside.

In a medium stockpot, combine stock, beets, mushrooms, mushroom liquid, celery leaves, parsley, garlic, sugar, a large pinch of salt, and a large pinch of pepper. Bring to a boil, reduce to a simmer, and cook for 20 minutes.

Add lemon juice or citric acid. Taste, and adjust for seasoning. In a small bowl, combine sour cream and flour, stirring until no lumps remain. Whisk in a ladleful of soup, then whisk the sour-cream mixture into the soup. Cook until slightly thickened, about 2 minutes. Stir in dill just before serving.