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To make the chermoula, put the oil, garlic, cumin, paprika, three-quarters of the coriander and the salt in a small bowl. Add the lemon juice, then blitz with a hand blender until smooth. Spoon half over the fish fillets and turn them over to coat both sides. Set aside to marinate.
Heat the oil and fry the onions and garlic until softened and starting to colour, about 4-5 mins. Add the cumin and paprika and cook for 2 mins more. Add the tomatoes, stock, olives and lemon zest, stir in remaining chermoula and simmer, uncovered for 10 mins.
Stir in the peppers and potatoes, cover and simmer for 15 mins until the potatoes are tender. If freezing, reserve half the sauce and chill before freezing.
Stir the remaining coriander into the tagine, then arrange the fish fillets on top (if freezing a batch, reserve 4 portions of fish), and cook for 4-6 mins until the fish is just cooked. Serve with basmati rice, boiled with a little saffron if you like.