Melt butter in a large pot. Add celery, onion and potato. Cover pot and stew vegetables over low heat for 10 minutes without browning. Add stock and herbs; cook vegetables until soft, about 30 minutes. Puree mixture in a blender, or force through a sieve. Stir in milk, adjust seasonings, toss in grated carrot and celery. Reheat to just below boiling point. Makes 6 servings.
