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1
Pound or blend the shallots, galangal, lemongrass, fresh chilli, and belacan to a fine, cohesive rempah paste, working until no fibrous threads of lemongrass remain, so that every part of the paste fries evenly and no pocket of raw galangal survives into the finished sauce. The paste should be smooth enough to move freely in the hot lard without catching and scorching at the edges.
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2
Score the pork belly skin at 1 cm intervals and cut the belly into pieces approximately 4–5 cm wide, scoring deeply enough that the fat beneath the skin can render out into the braising liquid rather than sitting sealed beneath a tight surface. Heat the lard in a heavy-based pot or wok over a medium flame, add the rempah, and fry, stirring constantly, until the paste darkens by a shade, the raw shallot smell has left, and the lard begins to separate at the edges — 5 to 7 minutes, because that fat weeping back out tells you the moisture is gone and the paste will not stew.
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3
Add the pork pieces and turn them through the paste until each surface is lightly coloured and the meat has taken on the fragrance of the rempah — 3 to 4 minutes, so the paste adheres to the pork and begins to perfume the flesh rather than simply sitting around it in the liquid. The pork need not be deeply browned; a pale, paste-coated surface is correct at this stage.
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4
Pour in the strained tamarind liquid and the additional water, then add the asam keping pieces so their slow, dry sourness has the full braising time to dissolve into the sauce and round out the sharper edge of the tamarind paste. Add the grated palm sugar and the tomatoes, stir to combine, bring to a gentle simmer, cover with a lid, and braise over low heat for 30 to 40 minutes, until the pork is beginning to yield and the skin is soft but still carries some resistance when pressed.
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5
Remove the lid — this is where the dish lives or dies: raise the heat to medium and reduce the sauce, stirring occasionally, for a further 15 to 20 minutes, because only at this point does the sauce tighten around the pork and turn from a broth into something dark, glossy, and clinging. The sauce must coat and cling to every piece without running freely onto the plate; a watery sauce, however correct in flavour, is an unfinished babi assam. Taste for balance and adjust tamarind, palm sugar, and salt so that no single element dominates.
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6
Taste the final sauce once more before serving: the sweet-sour-savoury triangle should be in equilibrium, the chilli heat gentle and lingering, the pork fat fully rendered into the sauce. Serve with white rice.