Place beans in a large saucepan with fresh water to cover. Bring to a boil. Cook about 1 hour until skins wrinkle. Drain, reserving liquid.
Place half of beans in bean pot. Add half of salt pork. Repeat. Mix sugar with next mustard, paprika and salt. Add chili sauce, tomato puree and reserved cooking liquid. Mix and pour over beans. Place onion on top.
Cover and bake at 325°F 2 hours, adding more cooking liquid if become too dry. Increase heat to 350°F and bake until tender, about 2 hours longer.