Place the beef in a large, heavy-based saucepan with the stock, cardamom, cloves, star anise, palm sugar, fish sauce, half the tamarind paste and half the coconut milk. Bring to the boil, reduce the heat and simmer gently for 2 hours, stirring occasionally.
Strain the beef over a large bowl, reserving the liquid. Cover and keep warm. Meanwhile, return the saucepan to a medium heat and add the curry paste and remaining coconut milk. Bring to the boil and add the remaining tamarind.
Stir in the beef with the lemon grass, shallots, butternut squash and 250 ml (8 fl oz) of the reserved liquid. Simmer, uncovered, for 20–25 minutes. Remove from the heat, season to taste and sprinkle over the spring onions just before serving.