To make the curry paste, place a dry frying pan over medium heat. Dry roast the cumin and coriander seeds, along with the mace and peppercorns until the cumin darkens. Remove from the pan and grind them to a powder in a pestle and mortar.
In a small food processor, blend the chillies with the shallots, garlic and lemongrass. Add the dry roasted spices, galangal, lime zest, coriander and shrimp paste. Blend to a paste. Fry the paste until fragrant, then stir in the coconut milk and sugar. Remove from the heat and set aside.
For the beef stir-fry, bring a pan of water to the boil and add the choy sum and gai lan. Blanch for one minute, then drain and refresh in ice-cold water. Chop the stems into bite-sized pieces and set aside with the leaves.
Meanwhile, heat a thick-based wok until very hot and then add the oil. Take the beef and drop it in so it sizzles. Once the beef is browned, add the lime leaves, chilli and choy sum stems with a good amount of the curry paste. Fry well so it goes sticky and sweet. Mix and then remove from the heat and add the herbs and the cooked choy sum and gai lan leaves.
Serve garnished with peanuts.