Heat butter or oil in a pan over medium. Sauté onion, carrots, and celery until just softened.
To a slow cooker, add onion-celery-carrot mixture, broth, pickle juice, pickles, frozen potatoes, frozen vegetables, salt, pepper, and sugar.
Cook on Low for 4 hours; until vegetables are soft.
Mix milk, sour cream, and flour in a small bowl until smooth. Add a small amount of hot soup broth to temper.
Stir the milk mixture into the soup until smooth. Do not allow the soup to boil or it can curdle.
Divide soup into bowls and garnish with fresh parsley and dill.
