Fish Stew with Rouille

In a deep frying pan or casserole, heat the remaining olive oil. Add the finely diced fennel, onion, and chopped garlic. Season them lightly, then cover the pan and cook gently for 10 minutes until the vegetables have softened. While that cooks, peel the potato and cut it into roughly 2cm chunks. Place the potato in a pan of cold water, bring it to a boil, and cook for 5 minutes until it's almost tender. Drain the potatoes in a colander.

Peel a strip of zest from the orange. Add the orange zest, star anise, bay leaf, and ½ tsp harissa spice to the pan with the softened vegetables. Fry gently, uncovered, for another 5–10 minutes, until the vegetables are soft, sweet, and lightly golden.

Stir in the tomato purée and cook for 2 minutes, letting it deepen in color. Then, add the chopped tomatoes and the strained prawn stock. Bring the sauce to a simmer and cook for 10 minutes, letting it thicken slightly. Season the sauce to your liking. (This sauce can be made ahead and reheated later.) While the sauce simmers, scrub the mussels and pull away any stringy beards. If any mussels are open, tap them sharply on the worktop; discard any that don't close after a few seconds.

If you made the sauce ahead, reheat it. Gently stir the par-cooked potato, chunks of white fish, and prepared prawns into the stew. Bring it back to a boil, then cover and gently simmer for 3 minutes. Scatter the cleaned mussels over the stew, cover again, and cook for another 2 minutes, or until all the shells have opened wide. Discard any mussels that remain closed. The fish should flake easily and the prawns should be pink all the way through. Finish by scattering fresh thyme leaves over the stew.

For a quick rouille, simply stir the remaining harissa spice into mayonnaise. Serve the hot stew in bowls, topping each with a generous spoonful of rouille; it will melt into the rich sauce. Make sure to have some crusty bread on hand to soak up all the flavorful juices.