Place the beans in a large bowl. Add enough water to cover by three inches. Let stand at least 10 hours. Drain and transfer to a large saucepan. Add just enough water to cover. Let simmer, adjusting the water level so the beans remain just covered, until the skins open readily, about 1 hour.

Meanwhile, whisk together the molasses, vinegar, mustard, sugar, pepper, and 2 tablespoons of water.

Drain the beans. Place the onion in a 2-quart Dutch oven. Add the beans. Cover with the salt pork or bacon. Cover with the molasses mixture. Heat oven to 300° F. Cover Dutch oven and bake, checking often, until the beans are tender, the meat is falling apart, and the liquid is thick and bubbly, about 6 hours. Add more water (or even a second batch of the molasses mixture) if the beans start to dry out. Serve with buttered bread.