1. 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.
2. 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.
3. 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.
Wine Recommendation: The lemon and mustard in the vinaigrette will work well with an acidic white wine that doesn't have a strong flavor. Try a pinot grigio or Soave from Italy.