Spread half of the salt mixture into a large rectangular shape on a jellyroll pan.
Place whole fish on top of salt; insert rosemary sprigs and lemon slices into cavity of fish. Pat remaining salt mixture over fish to cover completely.
Bake at 450 for 30 minutes.
Tap edge of salt with a knife to loosen; lift salt off fish.
Serve with lemon wedges and drizzle with extra-virgin olive oil, if desired.