Place beans in a large pot and cover with water by 2-inches. Soak overnight and drain the next morning.
Cover the beans with fresh water by 3-inches and simmer until tender, approximately 1 to 2 hours. Drain and reserve the liquid.
Place beans, salt pork, bacon, and onion into a 2 1/2-quart bean pot or covered casserole dish.
In a medium bowl, whisk together brown sugar, molasses, tomato paste, and mustard, until well combined. Pour over beans and add just enough reserved water to cover, stir well.
Preheat the oven to 325 degrees.
Cover and bake for 3-4 hours, until beans are tender. Remove the lid about halfway through cooking, and add more liquid if necessary to prevent the beans from getting too dry. Season with salt and pepper to taste.
Cook's Note: Beans can also be cooked in a Crock Pot on LOW for 6-8 hours.