Dry roast cumin, coriander seeds, mace, and peppercorns in a pan until fragrant, then grind to a powder.
Blend chillies, shallots, garlic, lemongrass, roasted spices, galangal, lime zest, coriander, and shrimp paste in a food processor.
Fry the paste until fragrant, stir in coconut milk and sugar, then remove from heat and set aside.
Blanch choy sum and gai lan in boiling water for one minute, refresh in ice-cold water, and chop stems.
Heat oil in a hot wok and sear the beef until browned.
Add lime leaves, chilli, choy sum stems, and 3 tbsp of curry paste to the wok and fry until sticky.
Remove from heat and stir in the herbs, blanched leaves, and garnish with crushed peanuts before serving.