Light the barbecue – you want an even layer of ashen embers for medium heat. If you have a gas barbecue, set it to medium. Season the fish cavities with salt and pepper, then stuff with most of the rosemary sprigs and lemon slices. Brush or rub the fish skin with the olive oil and season well with sea salt flakes. If you have a fish cage, lay the fish inside with the remaining lemon slices and rosemary sprigs and place on the grill. Alternatively, scatter the lemon slices and rosemary sprigs directly over the grill and lay the fish on top (see tip below for cooking indoors).

Cook the fish for 8-10 mins on each side until the skin is lightly charred and blistered, and the fish is cooked through. While it is cooking, put the lemon halves cut-side down on the grill and cook until char marks appear. Serve the fish drizzled with extra oil with the charred lemons for squeezing over.