Place the coriander stalks and roots in a small food processor or blender, reserving the sprigs to garnish. Add the curry paste, chilli powder, galangal or ginger, two of the lime leaves and garlic and puree to a smooth paste.
Pour the milk from the can of coconut milk into a jug leaving the cream behind.
Heat a wok to the smoking hot stage. Scrape the coconut cream out of the can and add to the wok. Quickly add the curry paste mixture and stir-fry for 2-3 minutes. Next add the beef and fry for a further 8-10 minutes until lightly browned.
Now add the coconut milk, the stock and the remaining lime leaves. Bring to a gentle the boil and then reduce to a gentle simmer. Cook for about 1 hour until the beef is almost tender. Stir in the the butternut squash and cook for another 20-25 minutes until the beef and butternut squash are cooked through and completely tender.
Finally stir in the tamarind or lime juice and the fish sauce. Serve in deep bowls with a sprinkling of spring onions, coriander sprigs and the steamed rice.