1.Shell and devein prawns, leaving tails intact.
2.Using mortar and pestle, crush coriander root and stem mixture, coriander seeds, peppercorns and garlic to a paste. In a large bowl, combine paste with prawns and half the oil. Cover; refrigerate 3 hours or overnight.
3.Heat remaining oil in wok; stir-fry prawn mixture, in batches, until prawns are changed in colour. Return prawns to wok; remove from heat, stir in sprouts and chopped coriander. Serve sprinkled with remaining ingredients.