How to Make It
Preheat the oven to 425°. In a small bowl, whisk 3 tablespoons of the olive oil with the pimentón and lemon zest. In a large bowl, mix the 6 cups of kosher salt with the egg whites and 1/2 cup of water until it resembles moist sand.
Spread a 1/4-inch-thick layer of the salt mixture in the center of a large rimmed baking sheet. Season the fish inside and out with black pepper and brush all over with the pimentón oil. Stuff the cavity with the lemon slices and bay leaves and lay the snapper on the salt. Mound the remaining salt mixture on top, lightly packing it to completely cover the fish.
Bake the fish for about 30 minutes, until an instant-read thermometer inserted into the fish through the salt registers 135°. Remove from the oven and let stand for 10 minutes. Crack the salt crust and discard it. Brush off any excess salt and transfer the fish to a platter.
In a large bowl, toss the celery with the celery leaves, parsley, tarragon, lemon juice and the remaining 2 tablespoons of olive oil. Season the salad with salt and pepper and serve alongside the fish.