Season the chicken breasts with salt and pepper. Heat the olive oil in a skillet over medium heat and cook the chicken for 6-7 minutes on each side, or until fully cooked through and golden brown. If using, drizzle with balsamic vinegar for extra flavor. Let the chicken rest for a few minutes, then slice thinly.
While the chicken is cooking, slice the ciabatta rolls in half and toast them in a toaster or on a griddle until golden and crispy.
Spread pesto on both sides of the toasted ciabatta rolls. Place a layer of sliced chicken on the bottom half, followed by a few slices of fresh mozzarella cheese (if using). Add a handful of arugula or spinach for a fresh crunch.
Top with the other half of the ciabatta roll, slice the sandwich in half, and serve immediately. Enjoy with a side of chips or a light salad for a complete meal!