01 - Heat a grill pan or large skillet over medium-high heat.
02 - Slice each chicken breast horizontally to create thin cutlets. Drizzle with olive oil and season both sides evenly with salt and black pepper.
03 - Arrange chicken cutlets on the hot pan and grill for 4 to 5 minutes per side, or until fully cooked through and golden. Remove the chicken from heat and let rest for 2 minutes. Slice if desired.
04 - Optionally, spread butter on the inside surfaces of the ciabatta rolls. Toast the cut sides on the grill pan or skillet until golden and crisp.
05 - Spread a generous layer of basil pesto on the bottom half of each roll. Layer sliced grilled chicken, fresh mozzarella, tomato slices, and baby arugula or spinach leaves. Cap each sandwich with the top half of the roll.
06 - Serve each sandwich warm or at room temperature.