Kaeng Som (Sour Curry with Fish and Vegetables)

Variations

• A paste fried in fat, and a paste dissolved in water. Every other curry here cooks its paste in fat first: green curry cracks coconut cream and fries the paste in the oil that separates, laksa fries its rempah 15-20 minutes until the oil breaks out, nasi lemak cooks the sambal paste "until the red oil separates". Blooming spices gives the reason in one line — aroma dissolves in fat, not water. Kaeng som has neither coconut milk nor oil, so that line simply does not apply to it.
• Which is why the pounding is long. With no fat to do the extracting, the only variable left is surface area, so this paste is worked until no fibre is visible at all. A blender does it badly: blades cut rather than crush, and crushing is what opens the cells (knife skills).
• Tom yum answers the same problem the other way. It is also fat-free and also sour, and instead of grinding its aromatics it bruises them whole — lemongrass, galangal, kaffir lime leaf — and infuses them in the broth. Two fat-free Thai soups, and the fork in the road is ground paste versus whole aromatics.
• Sinigang shares only the tamarind. The Filipino soup has no chilli and no paste at all and runs on sourness alone. Kaeng som is that sourness with chilli and kapi on top of it.
• Kaeng lueang (yellow sour curry): the southern version, with a generous amount of turmeric in the paste. Yellower, and much hotter.
• Cha-om kai: the best-known variation fries an omelette of acacia shoots, cuts it into squares and drops them in. It is the one version in which any fat reaches the bowl at all.
• With prawns: use prawns instead of fish and pound the shells and heads into the paste.

Tips

• Start with water, not stock. What the paste and the fish can give is what this dish tastes of, and a stock underneath blurs that outline.
• The tamarind goes in after the vegetables. Acid sets pectin firm, so vegetables cook far more slowly once it is in. It is the property soaking describes for beans and the one bigos turns to its own advantage; here it is only an obstruction, so the daikon and cauliflower go soft first. Add it early and the daikon is still hard after half an hour.
• The heat arrives differently. Chilli and heat explains that capsaicin dissolves in fat and not in water. In a coconut curry the fat spreads it evenly through the whole bowl; in kaeng som there is nothing to dissolve it, so it stays on the chilli particles suspended in the broth. The same weight of chilli reads sharper, and unevenly — spoonful to spoonful.
• Season off the heat, always. Fish sauce loses its aroma when boiled and tamarind sharpens as it reduces. And with no fat to soften them, salt and acid arrive undisguised, so your tongue is a better guide than the quantities above.
• Fish last, and stop stirring once it is in. There is no fat and no starch to hold this broth together, so fish broken up in it just turns it cloudy.