Use a mortar and pestle to pound the garlic and coriander roots to a rough paste. Mix the prawns with the garlic paste, pepper, fish sauce and sugar.
Heat enough vegetable oil in a saucepan for deep frying. When the oil is hot, toss the prawns in the flour and then cook in batches in the hot oil for 2-3 minutes or until golden and cooked through. Drain on paper towel. Serve with Coconut Sweet Chilli as a dipping sauce.