Dr. Weil

Pick over beans to remove any dirt, stones or foreign objects. Wash well, then soak for 8 hours in ample cold water.Drain beans and cover with a generous amount of fresh water. Bring to a boil over high heat in a large saucepan with the bay leaf.Skim off foam, lower heat, and simmer, partially covered, until beans are just tender, about 1 hour.Add onion and continue to cook until onion becomes extremely soft, about 1 more hour.Add salt to taste and garlic. Continue to cook, adding a little boiling water if necessary, until beans are very soft, about 1-2 hours more.Remove bay leaf and turn off heat.Ladle beans in batches into a blender or food processor and puree, or use an immersion blender and puree soup directly in the saucepan.Add dry mustard powder and dry sherry. Correct seasoning.Reheat and serve, adding any garnishes you wish, such as slices of lemon or freshly chopped herbs.