In a tagine, layer the sliced fennel, onion, tomato and preserved lemon and scatter with the fennel seeds. Mix all the ingredients for the paste together, season to taste and add a drizzle of olive oil.
Make three deep slashes on each side of the fish and two across to create diamond shapes. Rub the harissa paste into the slashes on each side of the fish as well as into the cavity.
Place the fish onto the fennel base, scatted over the olives, add the lemon juice, lemon zest, crushed fennel seeds, chillies and garlic and drizzle the fish with olive oil. Cover the tagine or baking dish and cook over a low heat or open fire for 24-35 minutes. Serve with fresh Moroccan bread.