1. Mix together, garlic, fish sauce, sugar, pepper, coriander root and cleaned prawn, then leave to marinate for 15-25 minutes.
2. Heat oil in a wok over medium heat. Add prawns and their marinade, then stir-fry for 2-4 minutes or until the color turns to white. (Tip : to get the best taste of prawns, it must not be over cooked.)
3. Transfer to a serving dish. Garnish with sliced tomatoes and cucumber, then serve immediately.