Rinse sorrel and shake dry. Rinse, peel and dice potatoes and celery root. Peel and finely chop shallots. In a pot, sauté shallots in butter. Add potatoes and celery root and sauté briefly. Add vegetable stock and simmer about 15 minutes. Rinse, trim and finely slice scallions crosswise.
Stir in up to 2 tablespoons sorrel. Puree soup and strain through a sieve into another pot. Add up to 4 tablespoons cream and bring to a boil. Continue cooking or add a little more broth until soup reaches the desired consistency. Season with salt and pepper.
Ladle soup into bowls and drizzle with remaining cream. Use a spoon to create a decorative pattern with the cream. Sprinkle soup with scallions and remaining sorrel (cut into strips).