The fish is wrapped in gleaming strips of leek before being baked in a salt crust--together, the two layers infuse the fish with hints of citrus and pepper, as well as just the right jolt of saltiness.
Preheat oven to 400 degrees. Prepare a large ice-water bath. Working in two batches, blanch leek in a pot of boiling water until just tender, about 2 minutes a batch. Transfer to ice bath to cool, then let drain on a kitchen towel.
In a large bowl, combine salt, peppercorns, and orange zest. Stir in egg whites until combined.
Stuff fish cavity with thyme sprigs. Starting just below head, wrap leek leaves around fish to completely cover, ending just above tail.
Spread half of salt mixture on a parchment-lined rimmed baking sheet about 1 inch larger than fish. Place fish on salt bed; top with remaining salt mixture, pressing to cover all but head and tail.
Bake until salt is golden and firm and the tip of a thin, sharp knife pierced through crust into center of fish is hot, 40 to 45 minutes. Using a knife and spoon, break salt crust and remove in sections. Remove excess salt with a pastry brush. With knife, cut fish into 8 even pieces along spine, then transfer to a plate. Remove and discard spine, then cut and transfer 8 pieces of fish from other side. Drizzle with hollandaise and serve, with remaining hollandaise on the side.