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1
Break the tamarind block into the warm water and work it with the fingers until fully dissolved. Pass through a fine sieve, pressing the pulp; discard the fibres and seeds. Taste the strained liquid — it should be sharply sour and clean, the sourness assertive enough to be the dominant note when diluted into 1.5 litres of broth.
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2
Bring the broth to a gentle simmer in a wide pot. Add the tamarind water, fish sauce, and sugar. Stir and taste: the broth must read sour first, then sweet, then savoury — this calibration, made before the fish enters the pot, is where the dish lives or dies; adjust tamarind, sugar, or fish sauce until the balance is unambiguous.
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3
Add the tomato wedges and return the broth to a simmer. Cook for 3–4 minutes until the tomatoes have softened but still hold their shape — they should yield at the edge when pressed with a spoon but not collapse into the broth.
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4
Lower the fish chunks into the simmering broth in a single layer. Cook at a bare simmer — the surface of the broth should tremble, not boil — for 4–6 minutes depending on thickness. The fish is done when it turns opaque through to the centre and begins to flake at the thickest point when nudged with a spoon; do not stir.
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5
Add the pineapple pieces in the final 5 minutes of cooking. They should warm through and just begin to soften at the edges — remove the pot from the heat the moment the pineapple loses its raw firmness; prolonged heat allows bromelain to continue breaking down the fish.
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6
Ladle the soup into deep bowls. Serve the bean sprouts, torn rice paddy herb (ngổ ôm), and sliced chilli in quantity alongside; each diner adds the garnish to their own bowl. The broth at the table should be vivid, clear, and fragrant with the cumin-citrus note of the ngổ ôm.