Heat oven to 450°F. Line a half sheet pan with parchment paper.
Place a few lemon slices and the parsley sprigs inside the cavity of the fish.
Combine the salt, egg whites, and 1/4 cup water in a large mixing bowl, working it together with your hands until it resembles wet sand and holds together when squeezed. Place about a third of the salt mixture on the sheet pan and shape it into an oval roughly the size of the fish and 1/2-inch thick. Lay a few lemon slices across the salt and park the fish on top.
Pile the remaining salt mixture onto the fish, patting it into a dome 1/2-inch thick. The end of the tail should stick out of the end of the dome. Depending on the size of the fish, you may not need all of the salt mixture.
Roast for 15 to 20 minutes, until the dome is lightly browned and the fish reaches an internal temperature of 130°F. Remove from the oven and let rest for 10 minutes.
Use a small hammer, meat tenderizer, mallet or rolling pin to crack the dome. The salt should come off easily in a few large slabs. Use a basting brush to brush any excess salt off the fish.
Use a fish knife or serrated pie server to make an incision all the way down the back of the fish and around the gill plate. Lift off the skin, working from head to tail. Remove the fillet in pieces from the top side of the fish. Once the top fillet is removed, gently remove the bone by pulling the tail up and forward. Remove the bottom fillet below, leaving the skin behind.