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1
Pound or blend the soaked dried chillies, shallots, galangal, and fermented soybean into a coarse paste, working them together long enough that the chilli breaks down to an even red mass with no large fibrous pieces. Keeping the texture coarse rather than smooth means the paste will fry unevenly if rushed — the fibrous bits scorch before the centre is cooked — so take the time here to get an even consistency. The paste is ready when it holds together and that uniform red colour runs throughout.
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2
Place the pork ribs in a large pot with the water, bring to a simmer, and skim the surface diligently until the liquid runs clear, so the finished broth carries a clean, rich pork character rather than a grey, murky one. Continue at a steady simmer — not a rolling boil, which clouds the liquid and toughens the meat — until the ribs yield at the bone without resistance. This is the collagen foundation of the soup; do not rush it.
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3
Add the drained, soaked dok ngiao flowers to the simmering broth, so their slight bitterness and tannic, earthy quality diffuse gradually into the liquid rather than hitting all at once as they would if added late. Simmer until the flowers have softened fully and the broth has taken on a distinct earthy note that bridges the chilli heat and the tomato's acidity — this is where the dish lives or dies; without it, the soup is not nam ngiao.
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4
Heat the oil in a separate pan over medium heat and fry the paste, stirring constantly, until it darkens by one shade and the raw shallot smell gives way to a smoky, roasted chilli aroma, so the paste loses its harsh edge and the oil around it turns deep red-orange. Frying the paste apart from the broth concentrates its flavour and allows the oil to carry the colour into the soup in a way that stirring raw paste directly into liquid never achieves. Do not let the paste catch and burn — keep it moving.
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5
Stir the fried paste into the pork rib broth and add the dried blood cake, so the mineral depth of the blood cake can integrate fully into the broth rather than sitting as a separate element. Simmer together until the broth is a uniform deep red-orange and no distinct raw paste aroma remains. The blood cake will firm slightly as it heats through; this is correct.
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6
Add the quartered tomatoes in the last 20 minutes of cooking, so they cook through and release their acidity into the broth without having time to collapse to a purée. They are done when they hold their shape and are still identifiable as tomato — their acidity is what lifts the heaviness of the pork and blood cake, so a tomato reduced to nothing has lost its purpose in the bowl.
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7
Divide the prepared rice noodles among four bowls and ladle the broth over, distributing the ribs, dok ngiao flowers, blood cake, and tomato evenly among them. The finished bowl should present a clear-ish, richly flavoured deep red-orange broth — not thick, not reduced to a sauce.