01 - Halve each chicken breast horizontally to yield 4 thin cutlets of even thickness.
02 - Toss the chicken cutlets with olive oil, salt, and black pepper in a bowl until evenly coated.
03 - Heat a grill pan or skillet over medium-high heat. Cook the cutlets for 3 to 4 minutes per side until golden brown and cooked through to an internal temperature of 165°F.
04 - During the final minute of cooking, lay a slice of fresh mozzarella on each cutlet and cover briefly to melt.
05 - Butter the cut sides of the ciabatta rolls and toast in a pan until golden, if desired.
06 - Apply a thin layer of mayonnaise to the bottom half of each roll, then spread 1 tablespoon of pesto over the mayo or directly onto the bread.
07 - Layer spinach or arugula, a slice of tomato, and the warm chicken with melted mozzarella on the bottom half, then cap with the top half of the roll. Serve immediately.