In a large saucepan, gently fry the leeks, onion, garlic, parsley, tomatoes, fennel, bayleaf, and cayenne pepper in the oil. Add the tomato puree and 2 litres cold water. Season with salt and pepper and sprinkle in the saffron. Add all the fish and cover with the potato slices. Bring to the boil and cook over a high heat until the potatoes are done. Serve with garlic croutons. (From Provence.)