Soak beans overnight in cold water to cover. (Drain or not, as you prefer.) Melt butter in soup kettle, and sauté celery and onions until soft, but not brown. Add stock, beans, and garlic. Cook, uncovered, 1-1/2 hours or until beans are tender. Press through food mill. Add salt and pepper to taste. Beat egg yolks and stir in sour cream. Add 1/2 cup hot soup to egg and cream mixture and stir until blended. Return mixture to kettle, stirring constantly. If soup is thicker than desired, thin with milk. Remove from heat and add herbs. Chill. Garnish with tomato or additional parsley.