Discard the woody ends from the lemon grass and chop. Place in a food processor with the lime leaves, onion, garlic, ginger, coriander, chilli flakes, salt, turmeric and measurement water. Process to a smooth paste, scraping down the mixture from the side of the bowl.
Heat the oil in a flameproof casserole and fry the beef in batches until browned on all sides, lifting out with a slotted spoon on to a plate. Tip in the spice paste and cook over a very gentle heat, stirring, for about 4–5 minutes or until most of the moisture has evaporated. (Add a dash of water if the paste starts to catch.)
Stir the coconut milk and sugar into the casserole and bring to a simmer. Return the beef to the pan, cover and cook in a preheated oven, 150°C (300°F), Gas Mark 2, for about 2½ hours or until the meat is very tender.
Remove the lid and return the casserole to the hob. Cook, stirring, for about 8–10 minutes until the moisture has evaporated and the sauce thickly coats the meat. Sprinkle with the chopped parsley and serve.