Preheat the oven to Gas Mark 6, 200°C, fan 180°C fan. Place the olives, garlic and tomato in a bowl. Pick the basil leaves from the stalks, shred finely and stir into the tomato mixture with the oil and vinegar, then season.
Place each frozen fillet on a sheet of nonstick baking paper then trim the edges of each sheet to shape into an oval. Spoon the mixture down the length of the fish then fold and crimp the paper so it encloses the fish in a parcel.
Place the parcels on a large baking sheet and bake for 30 minutes. Meanwhile, boil the potatoes for about 20 minutes until tender. Drain and mash, then set aside until the fish is cooked.
Open the parcels and serve on top of the mash, pouring over the cooking juices.
Cooking in a parcel ('en papillote') steams fish in its juice and works well with any fresh herbs or sauce.
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