Rinse the mussels and clams thoroughly under cold running water to help rid them of sand and grit. Scrub them well with a stiff brush under cold running water. Pull off and discard the ‘beards’ from the mussels. Tap any open molluscs with the back of a knife. If they refuse to close, discard them. Wash the molluscs again; set aside. Peel and de-vein the prawns, rinse, then pat dry. Cut each prawn crossways into 3 segments; set aside.

Cut the ginger crossways into 4–5 slices. Peel the onion. Combine the stock, ginger, whole onion and peppercorns in a pan and bring to a simmer. Cover and simmer gently for 15 minutes. Strain the stock into a clean pan. Add a little salt – allowing for the saltiness of the shellfish; keep hot.

Melt the butter in a wide, heavy pan. Add the flour and cook, stirring, over a low heat for 2 minutes, until bubbling but not brown. Slowly pour in the hot stock, stirring rapidly with a whisk. Simmer gently for 3–4 minutes. (If any lumps appear, strain at this stage.)

Add the mussels and clams to the soup and bring to the boil. Cover, turn the heat to medium and cook for 5 minutes, or until the shells have opened up. (If some have not, cook for another minute, then discard any that remain closed.) Put in the prawns, crab meat and coconut milk. Cook over a low heat for 1 minute. Serve the soup in wide soup plates, garnished with chopped parsley.