In a bowl, whisk together egg whites to get a foamy mixture. In a separate bowl, stir to combine the salts. Add in the egg whites and stir to combine using a spatula. Line a large baking sheet with parchment paper. Arrange half of the sea salt mixture in the center of the baking sheet to create a salt bed for the fish. Place a rack in the middle of the oven and preheat it to 200 °C / 390 °F.
Clean the fish, remove the gills and the entrails, leaving the flakes on. Add the herbs to the cavity of the fish and place the fish on the salt bed. Arrange the rest of the salt over the fish and close the fish with the salt completely. Press to close, and check the edges to see if they're sealed well.
Place the baking dish with the fish in the oven and roast for 18 - 20 minutes at 200 °C / 390 °F. Once the fish roasts, remove it from the oven and set aside for 10 minutes to finish the cooking process. Remove the salt crust and serve the fish with Gremolata sauce.