Heat the olive oil in a large casserole and gently sweat the onion, celery and garlic without colouring for about 10 minutes. Stir in the tomatoes and potatoes and cook for a further 3-5 minutes. Add the bay leaves, stock and saffron, stir and bring to the boil. Simmer for 15 minutes.
Add any fish only and cook gently for a further 15 minutes. Add any shellfish and cook for five minutes or until the prawns have turned pink and mussels have opened. If there is too much liquid, remove the fish and shellfish and bring the liquor to the boil. Simmer for another 10 minutes or so to reduce, then add the fish back in.
Squeeze in the lemon juice and stir in the herbs. Check the seasoning and serve with a drizzle of extra-virgin olive oil.
For the fish, use anything that doesn’t fall apart too easily, such as monkfish, snapper, John Dory, hake or grouper and raw langoustines or even mussels for extra flavour.