2 cups navy beans
1/2 pound slab bacon, cut into cubes
1 medium onion, peeled and chopped
1/3 cup molasses
2 teaspoons dry mustard
1 teaspoon ground black pepper
Soak beans in a large bowl of water for 6 hours or overnight. Drain beans and put them in a large oven-safe pot with a heavy bottom and a tightfitting lid. Add 1 teaspoon salt and enough cool water to cover 2 inches above the beans. Bring to a boil, then lower the heat and simmer gently, stirring occasionally, until the beans are just tender, approximately 30 to 40 minutes. Drain and remove beans.
Heat the oven to 250 degrees. Bring a kettle full of water to a boil on the stove. Return the heavy-bottomed pot to the stove and turn the heat to medium high. Cook the bacon in the bottom of the pot until it begins to brown, then turn off the heat and add the chopped onion and, on top of it, the beans. Mix together molasses, mustard and black pepper, and add the mixture to the pot. Pour in enough boiling water to cover beans, put the lid on and bake, occasionally adding more water to keep beans covered, until they are tender but not falling apart, 4 to 5 hours.
Remove beans from oven, uncover, stir and season with salt. With the lid off, return pot to oven and let beans finish cooking, uncovered and without additional water, until the sauce has thickened and the top is deeply crusty, about 45 minutes more.