To make the croutons heat oven to 200C/180C fan/gas 6. Lay the slices of bread on a flat baking tray in a single layer, drizzle with olive oil and bake for 15 mins until golden and crisp. Set aside – can be made a day ahead and kept in an airtight container.
Use a layer of the green part of the leek to wrap around and make a herb bundle with the thyme, bay, parsley stalks, orange peel and chilli. Tie everything together with kitchen string and set aside.
Heat the oil in a very large casserole dish or stock pot and throw in the onion, sliced leek and fennel and cook for about 10 mins until softened. Stir through the garlic and cook for 2 mins more, then add the herb bundle, tomato purée, star anise, Pernod if using, chopped tomatoes and saffron. Simmer and stir for a minute or two then pour over the fish stock. Season with salt and pepper, bring to a simmer, then add the piece of potato. Bubble everything gently for 30 mins until you have a thin tomatoey soup. When that piece of potato is on the brink of collapse, fish it out and set aside to make the rouille.
While the broth is simmering make the rouille by crushing the garlic, chilli and saffron with a pinch of salt in a mortar with a pestle. Mash in the cooked potato to make a sticky paste then whisk in the egg yolk and, very gradually, the olive oil until you make a mayonnaise-like sauce. Stir in the lemon juice and set aside.
Once the chunky tomato broth has cooked you have two options: for a rustic bouillabaisse, simply poach your fish in it along with the mussels, if you're using (just until they open) and serve. For a refined version, remove the herb bundle and star anise. Using a handheld or table-top blender, blitz the soup until smooth. Pass the soup through a sieve into a large, clean pan and bring to a gentle simmer. Starting with the densest fish, add the chunks to the broth and cook for 1 min before adding the next type. With the fish we used, the order was: monkfish, John Dory, grey mullet, snapper. When all the fish is in, scatter over the mussels, if using, and simmer everything for about 5 mins until just cooked and the mussels have opened.
Use a slotted spoon to carefully scoop the fish and mussels out onto a warmed serving platter, moisten with just a little broth and scatter over the chopped parsley. Bring everything to the table. Some people eat it as two courses, serving the broth with croutons and rouille first, then the fish spooned into the same bowl. Others simply serve it as a fish stew. Whichever way you choose the rouille is there to be stirred into the broth to thicken and give it a kick.