Grating the butternut squash into the soup is a little trick I saw in Sicily. It gives it good colour and sweetness.
Finely chop the onion, celery, fennel, garlic and chilli. Heat the oil in a large pan, add the onion, celery, fennel, garlic and chilli and sweat gently until soft. Add the wine, tomatoes or passata, squash and stock and bring to the boil. Cover and simmer gently for 30 minutes. Season and gently break up the tomatoes.
Roughly chop the salmon and halibut and add to the pan. Add the prawns or langoustine tails, cover and simmer for 10 minutes or until just cooked.
Taste the soup and season it again with salt and pepper and a squeeze of lemon juice, if necessary.
Serve drizzled with olive oil and sprinkled with the chopped parsley.