Clean the crab by removing the head and discarding the gills.
Cut the crab body into four pieces, split the claws, and halve the head while reserving juices.
Blend the red chillies, black beans, garlic, and warm oil into a paste.
Pound the ginger, garlic, coriander roots, turmeric, peppercorns, sugar, and bird’s-eye chilli in a mortar.
Separate and chop the greens and whites of Chinese chives and coriander.
Heat oil in a wok and briefly fry both prepared pastes.
Add the ginger and chive whites to the wok.
Toss the crab pieces in the sauce with coriander whites, sugar, fish sauce, and vinegar.
Stir in the reserved juices and herb greens, then garnish with remaining herbs.