In a small bowl, mix the soy and oyster sauces with the sugar and set aside.
Put the coriander roots, peppercorns and garlic into a small food processor and whiz to a paste.
Stir-fry the paste in a little oil in a wok or heavy-based frying pan for a minute or so, until it releases its fragrance. Add the prawns, sliced chilli and soy sauce mixture and sti