Smoky Caprese Sandwiches

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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