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1
Place the rib pieces in a pot, cover with cold water, bring to a full boil, and cook for 3 minutes — starting in cold water draws out the blood and impurities gradually, so they gather in the water rather than seizing into the meat. Drain, then rinse each piece under cold running water, working until the water runs clear and the surface of the ribs is clean of any grey scum.
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2
Return the rinsed ribs to the pot with 1.5 litres of fresh water and add the bruised lemongrass, galangal coins, and torn kaffir lime leaves — bruising and tearing the aromatics opens their fibres so the broth picks up their fragrance fully rather than just a faint trace. Bring to a gentle simmer and cook, uncovered, for 45–60 minutes; the ribs are done when they yield to pressure but still hold to the bone, not falling away.
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3
This is where the dish lives or dies: remove the pot from the heat entirely before seasoning, so the lime juice goes in cold enough to keep its sharp, raw brightness — cooking it even briefly flattens the sour edge that defines thom saap. Add the fish sauce, lime juice, and bruised bird's eye chillies, then taste immediately; the hot-sour register should be more assertive than tom yum, with the pork's depth sitting underneath, and you should correct fish sauce and lime juice only here, off the heat.
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4
Stir 1 teaspoon of toasted rice powder (khao khua) into each bowl just before ladling in the broth and ribs, so the powder blooms in the hot liquid and contributes its faint nutty depth without clumping or turning starchy if it sat in a pot. The broth should remain clear and thin — no cloudiness or heavy texture — with only the faintest thickening visible at the surface.
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5
Finish each bowl with a generous amount of fresh coriander and sliced spring onion. Serve immediately while the broth is at a full, visible steam.