First make the chermoula marinade. Blitz 3 tablespoons of oil with all of the chermoula ingredients and a good pinch of sea salt in a food processor. Pour half of it over the fish and let it marinate for 30 minutes to 1 hour.
In a large pan, heat 1 tablespoon of olive oil with the onions and ginger. Season and fry for 10 minutes until soft. Add the saffron and fry for another minute, then add the remaining marinade, chopped preserved lemon, tomato purée, stock and potatoes. Bring to a boil and then turn down the heat to a simmer for 10 minutes.
Add the fish and tomatoes to the tagine. Cook for 5 minutes or until the fish is cooked through. Taste and season with extra lemon and salt.
CHERMOULA MARINADE