1. Heat oil in a wok over medium heat. Add chopped garlic and coriander root, then stir fry until fragrant.
2. Add prawns and stir fry until nearly cooked. Season with fish sauce, sugar and pepper. Stir fry until all mixed well and remove from heat. (Tip : to get the best taste of prawns, it must not be overcooked.)
3. Transfer to a serving dish. Sprinkle on top with fried garlic. Garnish with sliced tomatoes and cucumber, then serve immediately.