Make the chermoula. Place the garlic, chilli and salt in a mortar and pound with a pestle to form a paste. Add the coriander leaves and pound to a coarse paste. Beat in the saffron, cumin, olive oil and lemon juice.
Transfer the chermoula to a large bowl, Add the fish chunks and stir to coat well. Cover with clingfilm and leave to marinate in the refrigerator for 1–2 hours.
Heat the oil in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the onion and celery and cook for 2–3 minutes until they begin to soften.
Stir in the preserved lemon rind and pour in the stock or water and wine or sherry. Bring to the boil, then reduce the heat, cover and simmer for 10–15 minutes. Season with salt and pepper.
Add the marinated fish and the remaining marinade to the tagine, re-cover and cook gently for a further 5 minutes until the fish is cooked through. Stir a little of the mint through the tagine and garnish with the remaining mint. Serve with plain couscous, if liked.