Firstly – cut fish into generous dice and coat in a mix of turmeric, chili powder and potato flour with a little salt. (To make cleaning up easy, arrange the pieces on the paper the fish came in, sprinkle half the mix over the top using a sieve, then flip all the pieces and repeat, then toss in remaining mix to fully coat on the other sides.) Deep-fry in a suitable oil in a wok until golden brown and firm on the outside, then rest on absorbent paper. (This step isn’t completely necessary, but helps the fish hold together later on, and adds a pleasantly meaty texture.)
Finely chop half a red onion, and add the other half to a blender or processor of your choice with garlic, lemongrass, galangal, shrimp paste and a ground spice mix (either grind from fresh or use ground spices) of cardamom, cinnamon, cumin, turmeric and star anise. If using tamarind pulp to make tamarind water for next step, soak in boiling water now.
Pour oil from wok into a suitable container, filtering with the sieve, for future use, and return a small amount to the wok after wiping out any solids. Fry the diced onion, with some whole fresh or dried chillies if you wish, until softened, then add the paste and continue to fry until very fragrant and the oil is beginning to separate out. Add the tamarind water, cook briefly til well combined, then add about half a tin of coconut milk and a sliced zucchini. Cook until the zucchini are tender and the gravy is thickening, then add the leaves of a few heads of bok choy, separated by cutting off the bases and well washed where the dirt tends to collect near the roots, and blanch for about a minute. Add brown sugar and fish sauce to taste. Add the fish pieces, fold through carefully as not to break them up, and some chopped spring onion, and heat until the fish is warmed through. Serve with jasmine rice.