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Preheat the oven to fan 170C/conventional 190C/gas 5. Boil potatoes in lightly salted water for about 12 minutes or until tender, then drain well.
Take 2 large sheets of foil, about 30cm square, and brush the middle area of each sheet with the olive oil. Put a fish fillet on top and spread with the tomato paste. Sprinkle with the lemon zest and juice, add the cooked potatoes, olives and capers and season with ground pepper.
Lay a sprig of rosemary or thyme on top then loosely wrap and secure each parcel tightly to completely enclose the ingredients. You can prepare these up to half a day in advance and keep them in the fridge.
Put the fish parcels on a baking sheet and bake for 20-25 minutes, or until the fish flakes when tested with a fork. Serve at once, with steamed green beans.