Soak the dry beans in the first measure of water overnight.
Stud the onion with the whole cloves.
Soak the salt pork in water to cover for 2 hours.
Drain the beans. Place them in a large kettle. Add water to reach 2" above the beans. Add the salt. Bring to a boil.
Lower the heat. Simmer until the beans are barely tender (30-40 minutes). Drain well.
Preheat the oven to 250x.
Place the clove-studded onion in the center of an earthenware casserole. Cover with half the beans (form a layer). Add half the salt pork (in a layer). Add the remaining beans (in a layer). Top with the remaining salt pork (in a layer).
Combine the sugar, rum, mustard, pepper and thyme. Add to the beans.
Add boiling water to barely cover. Cover the casserole tightly.
Bake until tender (4-5 hours), adding boiling water as needed to keep the beans moist. Uncover.
Cook 30 minutes longer - do not add any more water. Serve hot.