First, make a simple fish stock: put the fish head, skin and bones in a saucepan with all the ingredients for the stock. Add 1 litre of cold water and bring to a gentle simmer. Cook very gently for half an hour; do not allow it to boil hard, as this spoils the flavour. Strain the stock, discarding the fishy bits and vegetables.
For the soup, finely chop the garlic, onion, celery and leeks. Peel the potatoes and cut into 1cm cubes. Heat the oil in a large saucepan, add the fennel seeds, garlic, onion, celery and leeks, then cover and sweat for about 10 minutes, until soft. Add the potatoes and the stock and return to a gentle simmer. Cook for 15 minutes, or until the potatoes are tender.
Meanwhile, make the rouille, if you are serving it, by chopping the chilli very finely and mixing it with the garlic mayonnaise; set aside.
Now, slice the fish fillets into bite-sized pieces. If using squid, slice the pouches into rings; if using scallops, clean and slice horizontally in half. Add all the fish and any shellfish to the simmering broth. Cook for 2 minutes, until the fish is just done, then remove from the heat. Add the parsley and season to taste.
Serve the soup in warm bowls, topped with a few crisp croûtons and a good blob of rouille, if you like.
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