Clean and scale (but don't fillet) the sea bass. Preheat the oven to 400F/200C/Gas 6.
Make three diagonal slashes in the flesh of the sea bass on both sides of the fish, going right down to the bone (but not cutting through the bone).
Insert rosemary sprigs into the slashes and the belly cavity of the fish. Let it hang out a bit, as it will singe slightly, giving an extra flavour to the dish.
Place the fish in a roasting tray and sprinkle with sea salt. Drizzle with olive oil then rub the olive oil and sea salt into the skin of the fish.
Place the fish in the preheated oven and roast uncovered for 30-35 minutes. The flesh should be white, the skin crisp.
Carefully remove the fish fillets with a spoon.
Serve with a squeeze of lemon and a drizzle of extra virgin olive oil.
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