Place the beans in a deep bowl and cover them with plenty of cold water. Leave overnight to plump up.
The next day, drain the beans and put them in a large saucepan. Add the onion, carrot and bacon to the pan. Tuck in the bay leaves, the ham bone and the garlic.
Cover completely with water and bring to the boil. Skim any froth that appears on the surface and discard it. Turn the heat down to a simmer, cover then leave for an hour until the beans are almost tender.
Meanwhile, melt the duck fat in a second large lidded saucepan over a moderately high heat.
Add the cubed pork and fry it until its edges are golden. Remove with a slotted spoon and set aside.
Add the bacon lardons to the pan and fry until the fat is golden, then remove with a slotted spoon and add to the cubed pork.
Cut the sausages in half to give eight short pieces, then brown them in the fat as you did the pork and remove.
Cook the sliced chicken in the same way and remove with a slotted spoon, before doing the same with the corned beef.
Cook the sliced onions in the duck fat for a few minutes until softenened, but not coloured. Add the garlic and tomatoes to the pan, along with the bay leaves.
Return the meats to the pan and cover with water. Bring to the boil, then simmer for about 45 minutes covered with a lid, until the liquid comes to a stew consistency.
Preheat the oven to 160C/325F/Gas 3.
Drain the beans, reserving the cooking liquor and bacon. Discard the carrot and onion.
Cut the pork and its rind into thick strips. Chop the bacon that was cooked with the beans. Lay the meat in the bottom of a deep earthenware or enamelled cast iron casserole and cover with some of the beans.
Add a layer of the meat and onions, then another of beans. Tuck the pieces of duck in among the beans, then add more beans and meat until all is used up. Top up with any liquid from the meat and a splash of the cooking liquor from the beans.
Top with half of the breadcrumbs and transfer to the oven; cook for an hour.
Stir the cooked breadcrumbs into the cassoulet and top with the remaining crumbs. Drizzle a little of the bean liquor and a little more melted duck fat over the crumbs and return to the oven for a further half an hour or so, until the crust is golden.