How to Make It
Heat oven to 300° F. Cook the bacon in a Dutch oven over medium heat, stirring occasionally, until nearly crisp, 5 to 7 minutes. Add the onion, 1 teaspoon salt, and ½ teaspoon pepper and cook, stirring occasionally, until the onion is soft, 4 to 6 minutes more.
Add the beans, molasses, ketchup, mustard, and 5 cups water to the pot and stir to combine; bring to a boil.
Cover the pot and transfer to oven. Bake until the liquid is reduced by half, about 2½ hours. Uncover and bake, stirring once, until the cooking liquid is thickened, 1 to 1¼ hours more. Stir in the vinegar and serve.