If your ciabatta rolls are very bready, pick out a little of the crumb. Put the rolls on a baking pan and bake until warm and crisp on the outside, about 10 minutes.
Meanwhile, heat a small grill pan or cast-iron pan over medium-high heat until very hot. Toss the red onion in 1 tablespoon of the oil and sprinkle with a little salt, then grill, turning occasionally, until slightly softened and charred in spots, about 6 minutes.
Stir together the basil, smoked almonds, crushed red pepper, 1/4 teaspoon salt and the remaining 4 tablespoons olive oil in a bowl.
Arrange the bottom halves of the ciabatta rolls on a work surface. Drizzle with a little olive oil, then arrange the tomatoes on top; sprinkle with salt and pepper. Next, top evenly with the onions, followed by the almond mixture. Lay the mozzarella on top; sprinkle with salt and pepper and drizzle with a little more olive oil. Top with the arugula and serve immediately.