Place the spice paste ingredients, up to and including the chillies, in a food processor until roughly chopped, or pound using a mortar and pestle. Add the lemon grass and onion, and process or pound to a dry paste. Add the tamarind paste and blend to mix.
Heat the oil in a large saucepan over a medium-high heat. Working in batches, fry the beef for a few minutes until browned on all sides. Remove each batch with a slotted spoon and set aside. Add the spice paste to the hot pan and fry for 2–3 minutes, stirring constantly. Return the beef to the pan with all the remaining ingredients. Pour in 250 ml (8 fl oz) water, reduce the heat and bring slowly to the boil, stirring constantly.
Reduce the heat again to as low as possible and simmer very gently for 4–4½ hours, stirring occasionally, until the meat is tender and the sauce has reduced and thickened. Increase the heat and, stirring constantly, fry the beef in the thick sauce until it is a rich brown colour and nearly all of the sauce has been absorbed. Serve hot.