Process the spice paste ingredients up to and including the chillies in a blender or food processor until roughly chopped, or pound using a mortar and pestle. Add the lemon grass and onions, and process or pound to a dry paste. Add the tamarind paste and blend together.
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 on a plate. Add the spice paste to the hot pan, and fry for 2–3 minutes, stirring constantly. Return the reserved beef to the pan with all the remaining ingredients, reduce the heat slightly 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 – the flavour improves with slow cooking.
Increase the heat when the sauce is very thick to make a true 'dry' rendang, if liked. Stirring constantly, fry the beef in the thick sauce until it is a rich brown colour and almost dry, and nearly all of the sauce has been absorbed. Serve hot.