How to Make It
Heat the oven to 450°. Rinse the fish and dry the surface and the cavity thoroughly with paper towels. Put the fish on a baking sheet and cut shallow incisions about 1 inch apart in each side. Rub the fish all over with 1 tablespoon of the oil. Sprinkle both sides with the garlic, 1/2 teaspoon of the salt, and the 2 teaspoons lemon juice. Put the rosemary, if using, in the fish cavity. Roast the fish until just done, about 25 minutes.
Meanwhile, in a small glass or stainless-steel bowl, whisk together the 1 tablespoon lemon juice, the wine vinegar, the mustard, the remaining 1/2 teaspoon salt, and the pepper. Add the remaining 7 tablespoons oil slowly, whisking. Whisk in the parsley.
Serve the fish on a platter. Run a knife between the flesh and the bones and lift off the fillet with the skin attached. Turn the fish over and repeat. Pass the parsley vinaigrette.
Fish Alternatives You can use other medium whole fish with moderately firm white flesh, such as sea bass, blackfish, grouper, or tilefish, in place of the snapper.