Wash beans and add cold water. Cover and let soak overnight. Add salt to beans and liquid and simmer gently until tender, about 1 hour. Drain, reserve liquid. Measure 1-3/4 cup bean liquid, add water if necessary. Cut a slice from the salt pork and place in the bottom of the bean pot. Add beans; place the onion in center of the beans. Mix mustard, sugar, molasses, and maple syrup with bean liquid; pour over beans. Cut 3 gashes in the remaining salt pork, and place on top of the beans. Rind side up. Cover and bake 5 to 7 hours in a 300°F oven, adding more water as needed. Remove cover from bean pot the last half hour so pork rind will brown and become crisp.