Pick over the beans and discard any misshapen beans and stones. Rinse, place in a bowl and add water to cover generously. Let stand for 3 hours. Drain and place in a saucepan with the onion and water to cover by 2 inches. Bring to a boil, reduce the heat to low and simmer gently, uncovered, until almost tender, 40 to 50 minutes. Drain, discarding the onion and reserving the liquid. Preheat an oven to 350°F. In a large ovenproof stew pot over low heat, fry the bacon until it begins to turn golden, about 5 minutes. Using a slotted spoon, transfer to a plate. Reserve the drippings in the pot. Season the lamb and pork with salt and pepper and add to the stew pot. Place in the oven and roast, basting occasionally with the bacon drippings, until the meat is tender, about 1¼ hours. Remove from the oven, let cool and cut into 1-inch cubes. Set aside. Do not wash the pot. Meanwhile, prick the sausages with a fork. Place in a fry pan, add water to cover halfway and simmer gently, turning once, until almost cooked through, about 12 minutes total. Drain, let cool and slice on the diagonal. Place one-third of the drained beans in the reserved pot. Sprinkle with half of the bacon, lamb, pork, sausage, garlic, salt and pepper. Using kitchen string, tie the parsley stems, thyme sprigs and bay leaves into a bundle. Add to the pot. Repeat the layers, using half of the remaining beans and all of the remaining meats, sausage and garlic. Season with salt and pepper and top with the remaining beans. In a bowl, whisk together the tomato paste, allspice, the 1 tsp. salt, the ½ tsp. pepper and 2 cups of the reserved bean liquid. Pour into the pot just to cover the beans. Bake, uncovered, for 1 hour; add more bean liquid, if necessary, to prevent the beans from drying out. Sprinkle with the bread crumbs and continue to bake until golden, about 1 hour more. Discard the herb bundle and serve directly from the pot.