Begin by gently pounding the chicken breasts to about 1/4-inch thickness for even cooking. Spread 1 tablespoon of pesto over each piece, ensuring to coat it evenly. Season both sides with salt and freshly ground black pepper.

Preheat your grill to medium heat, around 350°F to 400°F. Carefully place the seasoned chicken onto the grill and cook for 3-5 minutes per side until the chicken reaches an internal temperature of 165°F.

While the chicken grills, slice your ciabatta buns in half and toast the cut-side down on a grill or griddle for about 2-3 minutes until golden brown and crispy.

Spread 3 tablespoons of pesto on the bottom halves of the ciabatta buns, layer on the roasted red peppers, followed by the grilled chicken. Finally, add slices of fresh mozzarella cheese.

Spread any remaining pesto on the cut sides of the top half of the buns. Place the tops on the assembled sandwiches and gently press down to secure all ingredients.

Serve immediately after assembly to maintain warmth and texture. Consider pairing with air-fried sweet potato fries or a light salad.