01 - Slice each chicken breast horizontally to create 4 thinner cutlets of even thickness for uniform cooking.
02 - Toss the chicken cutlets in a small bowl with olive oil, salt, and black pepper until evenly coated.
03 - Heat a grill pan or skillet over medium-high heat. Cook the chicken 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 top of each chicken cutlet and cover briefly to melt.
05 - If desired, butter the cut sides of the ciabatta rolls and toast them cut-side down in a pan until golden and crisp.
06 - Spread a thin layer of mayonnaise on the bottom half of each roll, then layer 1 tablespoon of basil pesto over the mayo.
07 - Layer baby spinach or arugula, a slice of tomato, and the warm chicken with melted mozzarella onto each bottom bun.
08 - Place the top half of each roll over the assembled ingredients and serve immediately while warm.