Gut and descale the fish, unless you have purchased it already clean. Rinse the fish and pat dry. Salt and pepper the cleaned cavity, then place one sprig of rosemary inside. Brush the outside of the fish with olive oil, then sprinkle with salt, pepper, and chopped rosemary.
Preheat the grill to high (450℉). Place the fish in an oiled fish basket, if you have one, or directly on the greased grill so it doesn't stick. Cook until the eyes turn white, and the skin is charred, about 6-8 minutes per side.
Place the fish on a platter and garnish with lemon slices or wedges.
Notes
Although this fish has few bones, figure about 1 lb. of raw, whole fish per person.