In a large pot, combine 6 cups of stock with carrots, potatoes, celery root, and parsnip. Bring to a boil over high heat. Reduce to a simmer, cover, and cook for 10–12 minutes or until vegetables are fork-tender.
While vegetables simmer, grate the pickles and set aside. In a frying pan, melt the butter over medium heat. Add garlic and bay leaves and cook until fragrant, about 1 minute. Add grated pickles, lower heat, cover, and cook for 8–12 minutes until soft.
Add the pickle mixture to the soup pot, followed by 1 cup of the pickle brine. Taste and season with salt and pepper. Add up to ½ cup more brine if you want a tangier flavor. Discard bay leaves.
(Optional) If using sour cream, temper it by mixing with cold water and then with a few spoonfuls of soup. Stir tempered cream into the pot. Simmer gently for 5 minutes.
Serve hot, garnished with fresh dill and black pepper.