Name: Kaeng Som (Sour Curry with Fish and Vegetables)
Prep time: 35 min
Cook time: 30 min
Total time: 1 h 5 min
Yield: 4 servings
Cuisine: Thai
Ingredients: 8 dried red chillies; 4 shallots; 4 cloves garlic; 30 g krachai; 1 tbsp shrimp paste; 0.5 tsp salt; 80 g white fish; 1000 ml water; 500 g white fish; 200 g daikon; 150 g cauliflower; 100 g green beans; 150 g cabbage; 5 tbsp tamarind water; 3 tbsp fish sauce; 1 tbsp palm sugar

Ingredients

4servings

Paste
dried red chillies · seeded and soaked 20 minutes in warm water8
shallots4
garlic4 cloves
krachai · fingerroot. If you cannot get it, 15g ginger stands in30 g
shrimp paste · Thai kapi1 tbsp
salt0.5 tsp
white fish · the paste's share. Poached separately80 g
Broth and vegetables
water · water, not stock1000 ml
white fish · firm-fleshed, such as sea bass or snakehead. Cut into large pieces500 g
daikon · in thick slices200 g
cauliflower · in bite-size florets150 g
green beans · cut into 5cm lengths100 g
cabbage · cut into large pieces150 g
Seasoning
tamarind water · pulp let down in hot water and strained5 tbsp
fish sauce3 tbsp
palm sugar · only enough to hold the sourness up1 tbsp

Steps

Seed the dried chillies and soak them in warm water 20 minutes, then squeeze dry.
⏲ 20 min
Poach the paste's 80g of fish in salted water for 5 minutes, lift it out and pick out the bones. This fish is the only body the dish has.
⏲ 5 min
Pound the salt and soaked chillies first in a mortar, then work in the shallot, garlic and krachai one at a time for 10 minutes or more, until no fibre is left and it is a smooth paste. This paste is never going to meet hot oil, so how fine it is is how much aroma you get. Add the shrimp paste and the poached fish last and pound smooth.
⏲ 10 min
Bring 1 litre of water to the boil and stir in all of the paste. There is no frying step, so there is no waiting for oil to separate. Boil 3 minutes to take the raw edge off.
⏲ 3 min
Add the daikon and simmer 8 minutes. The tamarind stays out for now — the reason is in the tips.
⏲ 8 min
Add the cauliflower and beans for 4 minutes, then the cabbage for 2 more.
⏲ 6 min
Slide in the fish, lower the heat and give it 4 minutes, until the flesh has just turned opaque. Stop stirring from here.
⏲ 4 min
Off the heat, add the tamarind water, fish sauce and palm sugar. Sour comes first, salt second, and sweet only holds them up. There is no fat to hide anything, so taste and adjust rather than measuring. Serve with rice.
⏲ 2 min

Variations

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.

More detail and the reasons why (4)

A paste fried in fat, and a paste dissolved in water. Most curries cook their 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". The reason is that aroma dissolves in fat, not water (blooming spices). Kaeng som has neither coconut milk nor oil, so that step simply does not exist in 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 paste at all — its green chillies go in whole, near the end — and runs on sourness. Kaeng som is that sourness with a pounded chilli-and-kapi paste on top of it.

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.
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.

More detail and the reasons why (3)

The tamarind goes in after the vegetables. Acid sets pectin firm, so vegetables cook far more slowly once it is in. It is the same property that keeps beans hard in acid (soaking), 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. Capsaicin dissolves in fat and not in water (chilli and heat). 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.

The basics behind this dish

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