Rinse the fish and dry well. Make several angled slits along the body of the fish, cutting down to the rib bones. Make two lateral slits along the back of the fish, from head to tail on either side of the dorsal fin. These cuts will ensure quick cooking and maximum crispiness.
Slowly heat the deep frying oil in a large saucepan. Heat the oil to around 175 degrees C. If you do not have a thermometer then dip the head of the fish into the oil; if it sizzles, it is ready, if it does not sizzle then wait a couple minutes and try again.
When the oil is hot enough slip the fish into the oil and fry for several minutes until it is crispy. Gently take the fish out of the oil and let it drain on paper towels.
Sauce: Fry the garlic, onion and chillies in a wok with oil. When the ingredients are lightly browned, add the fish sauce and soy sauce. Turn off the heat. Quickly mix in the basil and coriander.
To Serve: Place the fish on a large platter and pour the sauce over the fish. Serve with steamed jasmine rice.