Dry-fry the saffron in a small frying pan over a medium heat for less than a minute until it emits a faint aroma. Using a pestle and mortar or spice grinder, grind to a powder, then stir in the measurement water until the saffron dissolves.
Heat the oil in the base of a tagine or a large, heavy-based saucepan. Stir in the preserved lemon rind, fish, most of the mint and the saffron water and season with salt and pepper. Bring to the boil, then reduce the heat, cover and cook gently, stirring occasionally, for 15 minutes until the fish is cooked through. Garnish with the reserved mint and serve with couscous, if liked.