There are three main types of haricot bean: firstly, the long, thin, shiny-ivory one that the Italians sometimes call cannellini. These are delicious in salads, stews, and Tuscan bean and pasta soup; they are also the kind used in France, cooked in the Breton way and served with mutton and lamb.
The second type of haricot is the small round one of tinned baked bean fame. I think they are inferior to, and lack the flavour of, their handsomer cousins.
A member of the same family, tiny green unripe flageolets have a wonderful flavour.
Haricot beans are cheap, they help to bulk out meat and they are also very nutritious and filling. The other advantage is they are very good at absorbing the flavours of the dish. If you’re serving these, there’s no need to serve either rice or potatoes.
Neck fillet of lamb provides very sweet meat that responds perfectly to long, slow cooking and if you add pre-soaked dried green flageolets to cook alongside it these, too, absorb the sweet flavours of the lamb.