Heat the oil in a very large casserole dish or stockpot and put in the onion, sliced leek, tomatoes, pepper, fennel and cook for about 10 minutes until softened. Stir through the garlic and cook for 2 minutes more, then add the orange rind, tomato purée, star anise, pernod if using. 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 minutes 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.
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 for approximately 10-12 minutes, 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. Finish with salt and pepper and add in torn basil.