01 - Slice each chicken breast horizontally to create 4 thinner 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, place a slice of fresh mozzarella on each cutlet and cover briefly to melt.
05 - Butter the cut sides of the ciabatta rolls and toast them in a pan over medium heat until golden, if desired.
06 - Spread a thin layer of mayonnaise on the bottom half of each roll, then layer 1 tablespoon of basil pesto over the top.
07 - Arrange a handful of spinach or arugula and a slice of tomato on each bottom bun, then place the warm chicken with melted mozzarella on top.
08 - Cap each sandwich with the top half of the roll and serve immediately while warm.