Layer the potatoes, onion, celery and carrot in a 3- to 4-quart slow cooker. Sprinkle with the salt, thyme and pepper; top with the chicken broth. Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours or until the vegetables are tender. Using a potato masher, crush the vegetables slightly to thicken. Stir in ¼ cup of cream and lemon juice. Serve dolloped with the remaining sour cream and desired toppings.