Method

1. Preheat the oven to 170C/gas 3.

2. Pour the oil into a wide frying pan - enough to cover the base. Heat to a medium temperature and then brown the lamb shanks - about a minute on each side. Transfer the meat to a casserole dish.

3. Cook the onion, carrot and celery gently in the pan, until tender and lightly browned. Add the garlic and cook for a minute longer. Tip the vegetables into the casserole dish with the lamb. Add the flageolets and bouquet garni.

4. Tip any excess fat out of the frying pan and pour in 900ml of water. Bring to the boil, stirring and scraping the meat sediment from the bottom of the pan. Pour this over the casserole and season with pepper.

5. Transfer to the oven for 1-1½ hours - until the beans are meltingly tender, but not quite on the point of collapse.

6. Stir in the tomatoes, tomato puree, chilli and sugar, and season with salt. The tomatoes and salt prevent the beans from softening - this is why you only add them at this point. Continue cooking for a further hour, or until the meat is so tender it practically falls off the bone.

7. Remove the chilli from the pan. Take a couple of ladlefuls of the beans and juices, and blend in a food processor until smooth. Stir them back into the casserole - this lends a creamy texture to the finished casserole. Taste and adjust the seasoning if necessary.

8. For the gremolata, mix and finely chop the lemon zest, garlic and parsley. Sprinkle over the casserole just before serving.