Preheat the oven to 180°C, 350°F, Gas Mark 5.
Create four envelopes out of foil, by folding a large piece in half, then double folding over each side to seal and leaving the top edge open. Divide the prawns between the parcels.
Using a vegetable peeler, shave the asparagus and courgette into ribbons then add to the parcels. Segment the grapefruit by running a small knife down the fruit under the pith but leaving the flesh intact. Then run the knife between each membrane to loosen then segment – do this over a bowl – then add the segments to the parcels.
Sprinkle over the pepper or paprika then add a little oil and salt. Bake the parcels in the oven for about 8-10 minutes until the prawns are cooked and pink. Open one to check and return to the oven for another two minutes if necessary. Open the parcels at the table.
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