1. Process onion, oil and juice until smooth. Season with salt and pepper. Transfer to a large bowl. Add fish. Toss to coat.
2. Place four large pieces of foil on a flat surface. Top with a smaller piece of baking paper.
3. Place fish and onion mixture in centre of each piece of paper. Divide lemon slices, rosemary leaves and tomatoes over top. Fold one side of foil over the other side to enclose fish. Fold in ends tightly to form a parcel.
4. Cook foil parcels on a heated grill plate, on one side only, for 10 minutes, or until cooked through.
5. Meanwhile, to make lemon caper butter, melt butter in a small frying pan over a low heat. Add rind and capers. Gently heat through.
6. Carefully open fish parcels to allow steam to escape. Serve fish on parsley mashed potatoes. Drizzle generously with lemon caper butter. Serve with lemon wedges and garnish with rosemary.

Tip: To make parsley mashed potatoes, boil 1kg chopped potatoes until tender. Drain. Mash with 50g butter and 1 / 4 cup chopped fresh parsley. To cook fish in the oven, place parcels on an oven tray. Cook in a hot oven (200C) for about 15 minutes, or until cooked through.

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