Smoky Caprese Sandwiches

Name: Smoky Caprese Sandwiches
Ingredients: 4 sandwich-sized ciabatta rolls, split through the middle; 1 large red onion, cut into 1/4-inch-thick rings; 5 tablespoons olive oil, plus more for drizzling; Kosher salt; 1 cup loosely packed fresh basil leaves, cut into chiffonade; 1/2 cup smoked almonds, chopped; 1/4 teaspoon crushed red pepper flakes; 3 large heirloom tomatoes, preferably mixed colors, sliced 1/4 inch thick; Freshly ground black pepper; 1 pound fresh mozzarella, sliced 1/4 inch thick; 1 cup arugula