Scrub the mussels thoroughly under cold running water. Pull off the hairy 'beards' and rinse again. Gently tap any open mussels and discard any that do not close.
Sprinkle the water chestnuts with the sugar and pepper and set aside.
Heat the oil in a wok over a high heat until the oil starts to shimmer. Add the onion and mussels and stir-fry quickly for 1 minute. Put a lid on the wok and cook for 3–4 minutes or until the mussels have opened. Discard any mussels that remain closed.
Add the water chestnuts, prawns, spring onions, chilli flakes, sugar snaps and bean sprouts to the wok and stir-fry for 1–2 minutes or until the prawns have turned pink and are cooked through.
Mix together the soy sauce, yellow bean sauce and rice wine and pour over the ingredients in the wok. Stir-fry for a further 1–2 minutes until hot. Garnish with crushed chilli flakes and chervil sprigs. Serve with rice, if liked.