Wash and pick over beans. Place in slow cooker. Add water, salt and onion. Mix well. Cook on low 13-15 hours, or until beans are tender. Drain beans, reserving liquid. Add molasses, ketchup, brown sugar and mustard; stir. Return 2 cups of bean liquid and blend well. Cook on low 3-4 hours.

Note: Cooking time can be reduced if beans are soaked overnight before cooking. If you do this, discard soak water, then cook the beans in 9 cups of water on low for 8-10 hours.