For tom yum paste, drain chillies from soaking water; reserve 60ml soaking water. Transfer chillies to a food processor; add garlic, shallot, lemongrass, galangal, lime leaf and soaking water, then process to a smooth paste. Makes ¾ cup. Paste can be stored in an airtight container for up to 1 week.
Heat 1 tbsp oil in a wok or large frying pan over high heat. Stir-fry prawns until golden and just cooked through (2-3 minutes). Remove from pan. Heat remaining oil in wok until hot. Stir-fry mushrooms until golden and tender (2-3minutes). Add tom yum paste and stir-fry until fragrant (1-2 minutes). Add rice and stir-fry, breaking up clumps with a spoon, until grains are separated
(2-3 minutes). Stir in sugar, fish sauce, soy sauce and lime juice. Return the prawns and stir-fry until heated through.
Transfer rice to a serving bowl, scatter with coriander leaves and serve with baby cucumbers and lime wedges.
