Cut the piece of belly pork lengthways into three thick slices, then cut each piece across into two.
Preheat the oven to 180C/350F/Gas 4.
Heat 50g/1¾oz of the duck fat in a six-litre flameproof casserole dish.
Add the garlic and onion and fry gently until soft but not browned.
Add the beans and the pieces of salted belly pork, cover with 1¾ litres/3 pints water and push in the bouquet garni.
Bring to the boil, skimming off any scum as it rises to the surface, then cover, transfer to the oven and bake for one hour or until the beans are just tender (this will depend on the age of your beans).
Heat the remaining duck fat in a frying pan and brown the sausages all over.
Lift them onto a board and slice each one sharply on the diagonal into three pieces.
Remove the cassoulet from the oven and increase the oven temperature to 220C/425F/Gas 7.
Add the sausages and the pieces of duck confit to the casserole and push them down well into the beans.
Return the casserole to the oven and bake uncovered for a further 45 minutes or until the liquid has reduced and the cassoulet is covered in a dark golden crust.
Serve straight from the pot at the table.