Put the beans into a nonmetallic container, cover them with water, and soak them overnight. Drain the beans and put them into a cast-iron pot. Cover with fresh water, bring to a boil, and simmer for 1 hour. Drain the beans.
Preheat the oven to 250°F and put on some water to boil. Chop the onion and put it into the bottom of a large cast-iron pot. Add the beans to the pot. Mix the molasses, mustard, and salt, then spread this mixture over the beans. Put the slab of salt pork on top of the beans so that the rind is up. Cover the beans and pork with boiling water. Put the lid on the pot and bake for 8 hours. Add a little water from time to time if needed, but do not stir, leaving the slab of pork on top.