Heat the butter and oil in a large saucepan, add the onion and fry for 3 minutes until softened. Add the bacon and fry for a further 3–4 minutes until beginning to brown.
Add in the cornflour and cook, stirring, for 1 minute, then gradually add the stock and milk and cook, stirring all the time, until thickened and smooth. Mix in the potatoes and simmer for 10 minutes, then add the fish and cook gently for a further 5–6 minutes until just cooked through.
Mix in the prawns and cream and cook until the prawns are cooked through and piping hot. Season well, ladle into bowls and serve sprinkled with the parsley.