Preheat the oven to 200C/400F/Gas 6.
Scale and gut the fish (or you can ask your fishmonger to do this for you).
Place a layer of sea salt in the bottom of a roasting tin large enough to hold the fish comfortably.
Dry the scaled, gutted fish with kitchen paper. Stuff the body cavity with fresh rosemary sprigs.
Lay the fish on top of the salt, then cover the fish with the remaining sea salt. The fish should be completely enclosed by the salt. Sprinkle a bit of water on top of the salt (this will help it to form a crust).
Place the roasting pan in the preheated oven and cook for 25 minutes.
Remove the roasting pan from the oven. Break the salt crust with a palette knife. Using a pastry brush, remove the salt crystals from the surface of the fish and from around the fish.
Using a fish slice, carefully remove the fish from the salt and place onto a serving plate. Carefully remove the fish skin and fins.
Serve with lemon wedges.
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