Whole Branzino Roasted in Salt

INGREDIENTS

• 4 thin lemon slices
• 3 cups arugula
• 2 1- to 1 1/2-pound whole branzino, loup de mer, or sea bass, gutted
• 1/4 cup finely chopped fresh parsley
• 1/4 cup finely diced celery
• 1 3-pound box coarse kosher salt
• 1/4 cup extra-virgin olive oil
• 2 tablespoons drained small capers
• 1/4 cup finely diced cucumber
• 2 tablespoons sliced pitted brine-cured green olives (such as picholine)
• 1 lemon
• 8 fresh parsley sprigs
• 5 (or more) large egg whites
• 2 fresh thyme sprigs