Use a pestle and mortar or small blender to pound or blend the fresh chillies, garlic, shallots and coriander roots, forming a rough paste.
Heat 1½ tablespoons of the oil in a wok or large frying pan and stir-fry the chilli paste over a medium heat for 3–4 minutes until fragrant. Add the sugar or honey, fish sauce, tamarind purée or lime juice and simmer for 3–4 minutes or until the sugar has dissolved. Taste and adjust the seasoning. Remove from the heat.
Dust the fish with the flour.
Heat 10 cm (4 inches) of oil in a large wok over a medium heat. The oil is ready when a piece of garlic sizzles immediately when dropped in. Deep-fry the fish on one side for 10–12 minutes or until lightly browned. Drain on kitchen paper and keep warm. Deep-fry the remaining fish. Put the fish on a serving plate.
Spoon the warm chilli sauce over the fish and garnish with coriander leaves and chilli slices.