Set the oven to Gas Mark 3 or 160°C. Heat 1 tablespoon oil in a large pan, add the lamb shanks, 3 at a time, and brown them all over, as best you can. Remove and put into a large roasting tin.
Add bacon to the pan and cook for a few minutes, then add the rest of the oil, the onion, garlic and rosemary, and cook for 5 minutes. Add tomato purée, bay and thyme and, after a minute, add wine and simmer until reduced by half. Pour in stock and bring to the boil.
Spoon mixture over lamb shanks and cover tin with foil. Cook in oven for 2½ hours until lamb is falling off the bone. (Or cook in a flameproof casserole on the hob for the same length of time.)
Leave casserole to cool for an hour or so, then take meat off bones, discarding any fat and sinew, and bones. Shred meat into chunks and put in a container. Take bacon and onion out of sauce using a draining spoon, and add to meat. Put sauce in another container. Chill overnight or until ready to reheat.
When ready to reheat: Heat a little oil in a frying pan. Brown sausages for 10-15 minutes. Remove from pan and cut each one into 5 chunks.
Meanwhile, spoon sauce into a large pan and reduce it by about half. Add lamb, sausage and beans, and simmer for 10 minutes or until heated through.
For the garnish: Melt butter in a frying pan, add breadcrumbs and cook for about 5 minutes, stirring, until crisp and golden. Stir in the parsley, and scatter over the cassoulet for serving.