To make the curry paste, soak the dried chillies in water for 15 minutes, or until soft. Drain and squeeze any moisture out of the chillies, then chop finely. Place all the curry paste ingredients into a mortar and pestle and pound to a paste. Alternately, purée in a blender.
Bring the stock or water to the boil with the salt. Add the curry paste and simmer for 1 minute, then add the red onions.
Simmer for 3–4 minutes, then add the palm sugar, fish sauce and tamarind paste. Add the fish and pak choi and simmer until the fish is cooked through.
Top with the dry-roasted chillies, if using, and serve with steamed rice.
Cook's note: This brothy curry should taste sour, salty and a little hot. Adjust accordingly with fish sauce, tamarind paste and chilli powder