Place the potatoes in a large saucepan; cover with cold water. Bring to a boil, add 2 teaspoons salt, reduce heat, and simmer until tender, 15 to 18 minutes. Reserve 1/2 cup of the cooking water; drain the potatoes and return to the pot. Mash with the sour cream and 1/4 teaspoon pepper, adding some cooking water if the potatoes seem dry. Meanwhile, heat the oil in a large pot over medium heat. Add the onions, celery, and parsnips, season with 1/2 teaspoon salt and 1/4 teaspoon pepper, and cook, covered, stirring occasionally until very tender, 10 to 12 minutes. Add the garlic and cook, stirring, for 2 minutes. Add the thyme and 4 cups water and bring to a boil. Stir in the mashed potatoes and simmer until heated through, about 2 minutes. Remove the thyme. Using an immersion blender (or a standard blender, working in batches), purée the soup. Sprinkle with additional fresh thyme, drizzle with olive oil and serve with bread, if desired.