Preheat the oven to 400 degrees F.
Combine the kosher salt, parsley, rosemary, orange zest, orange juice, and black pepper in a large mixing bowl. Mix well. Line a baking sheet with parchment paper. Lay the rosemary stems then the orange slices. Place the fish on the orange slices and coat with the olive oil. Mound the salt mixture evenly on the surface of the fish, leaving the head and tail uncovered. With your fingers, firmly press the mixture into the flesh. Bake for 25 to 30 minutes.
Remove from the oven and cool for 2 minutes. With the back of a heavy spoon or a mallet, lightly pound the salt crust to crack it open, beginning at the tail end. Carefully pull off the salt crust.
To serve, use a small spatula or wide knife, remove the flesh from the bone from the topside of the fish. Remove the backbone, and then serve the flesh from the bottom side of the fish.