Preheat your oven to 200 degrees Celsius. If you look at the head and the tail of your fish, more often than not there's a secret line between them that the Big Man upstairs has drawn. Using this as your guide, carefully cut into one side of the line near the head, push down gently, angle the tip-end of your knife towards the bone and score between the flesh and the bone to peel away that beautiful fish fillet. Run the knife down to just above thet tail and part the fillet from the bone; about 4 to 5cm deep on both sides. Even if you don't get it perfect, you'll be stuffing this pocket with prawns so no one will know if your knife work was a bit shabby.
Get a roasting tray that snugly fits your fish and sprinkle your finely sliced onions around the base of the tray. Season both sides of your fish with salt and pepper and lay it on top of the onions. Try to sweet most of the onions under the fish so they sweeten as they cook. Put the butter into a small pan on a low heat, and once it's melted pour it into a bowl and leave to cool for 5 minutes.
Add a pinch of salt and pepper, the grated garlic and a pinch of cayenne to the butter, then grate over the zest of half your lemon. Toss the peeled prawns through this mixture until nicely coated, then stuff them loosely inside the fish, pouring over any flavoured butter left behind in the bowl. Before putting it into the oven, drizzle over some olive oil and a splash of white wine, then halve your lemon and add both halves to the tray. Adjust the cooking time depending on the size of your fish: a large fish will want 25 minutes, 2 small fish about 12 minutes. You'll know it's beautifully cooked when the flesh flakes away from the bone.
Finely chop the parsley leaves and sprinkle them over the fish once it's out of the oven. Squeeze over the juices from your roasted lemon halves, and serve. I like to put this in the middle of the table with something propping up one end of the tray so that the delicious milky juices run out of the fish and mingle with the butter, olive oil and lemon juices at one end of the tray. Spoon this over clumps of your fish and prawns, and anything else you're serving it with, like new potatoes, mash or simple steamed greens; it will taste wonderful.
Recipe by: Jamie Oliver (copyright), photography David Loftus (copyright)