Step 1Heat the oven to 400 degrees. Clip the fins off the fish and stuff its cavity with thyme.
Step 2Using your hands, in a large bowl mix together salt and egg whites until the salt looks and feels like wet sand.
Step 3Spread a bed of the salt mixture on a sheet pan or roasting pan, lay the fish on top, and then mold the salt mixture around it tightly, sealing the fish entirely.
Step 4Place the fish in the oven and roast until a thermometer poked through the crust and into the fish registers an internal temperature of 120 to 125 degrees, about 30 minutes.
Step 5Using two spatulas, gently lift the fish out of its salt bed and onto a clean work surface (it will be tender and want to fall apart easily). Scrape off any salt clinging to the fish with the blade of your slicing knife or with a pastry brush. It's important to be as thorough as possible with this.
Step 6Fillet the fish by slicing diagonally near the gills, and then straight down the middle of the back. Fold back the skin and lift the fillets off one at a time, followed by the skeleton, then retrieve the bottom fillets. Do a final check to make sure there are no bones or clumps of salt clinging to any of the fish portions. Serve immediately.