Preheat the oven to 220C/425F/Gas 7.
Slash the skin side of the sea bass diagonally so that the flesh is exposed.
Put a generous amount of the rosemary into the pockets.
Rub the rest of the fish with olive oil and sprinkle with salt.
Sear in a pan skin-side down until the skin starts to crisp. Transfer to the hot oven for five minutes.
Serve with a drizzle of fresh lemon and good olive oil.