01 - Slice each chicken breast horizontally to create 4 thinner cutlets for even cooking.
02 - Toss chicken with olive oil, salt, and pepper in a small bowl until evenly coated.
03 - Heat a grill pan or skillet over medium-high heat. Cook chicken for 3-4 minutes per side until golden brown and cooked through to an internal temperature of 165°F.
04 - Place a slice of mozzarella on each chicken piece during the last minute of cooking and cover briefly to melt.
05 - Butter the cut sides of the rolls and toast in a pan cut-side down until golden, if desired.
06 - Spread a thin layer of mayonnaise on the bottom half of each roll, then layer 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. Top with the remaining roll half and serve warm.