Preheat oven to 450 degrees F. Position a rack in the middle of oven. In a bowl, stir together salt and one cup water until salt is evenly moistened. Score both sides of the fish, cutting slits just through skin to expose a little flesh. Sprinkle inside cavity lightly with salt and a few grinds of black pepper. Rub two tablespoons of the herbs into cavity and insert lemon slices. Rub one tablespoon chopped herbs onto each side of the fish, pressing into slits.
Spread half the salt mixture in an oval just larger than the fish on a large baking sheet. Set fish on top of the salt. Pat remaining salt mixture over fish to cover completely and mold around curves of the fish. It's okay for the tail to stick out a little bit. Bake for 30 minutes. Remove baking sheet from oven and rap edges of salt crust to loosen it. Lift the crust from fish. It's okay if the skin comes away with crust.
To serve, use a spatula to cut exposed side of fish into two portions and place on two warmed dinner plates. Lift tail to remove bones in one piece and expose other side. Cut that side into two portions, lift it from bottom salt and place on warmed plates. Drizzle each portion with extra virgin olive oil and splash of sherry vinegar if desired. Sprinkle with remaining herbs and serve immediately.