1. Preheat oven to 400°. Stuff fish with sliced lemons and rosemary.
2. Combine kosher salt, egg whites, and 1/2 cup water in a large bowl. Line a large, rimmed baking sheet with aluminum foil or parchment paper. Spread one-third of the salt mixture on the lined baking sheet. Top with fish; cover with remaining salt mixture.
3. Bake fish 40 to 50 minutes. Crack salt layer with a hammer or meat mallet, and remove. Peel back fish skin; carefully remove meat, discarding skin and bones.