Vintage Recipes--Black Bean Soup

Cover 2 cups black beans with cold water; soak overnight. Drain; measure liquid. Add water to make 2 quarts; pour into kettle. Add beans, 1 large onion, sliced, 1/2 cup diced celery, 1 tsp salt, 4 peppercorns. Cover; simmer 3 hours, or until beans are tender; add water if necessary. Put through food mill or sieve. Add 1/4 tsp dry mustard. Blend 3 Tbsp butter and 1 1/2 Tbsp flour; stir into soup slowly. Heat thoroughly. Garnish with lemon slices. Makes 6 servings.