01 - Preheat a grill pan or large skillet over medium-high heat.
02 - Horizontally slice chicken breasts into thin cutlets. Drizzle both sides with olive oil, then season evenly with salt and black pepper.
03 - Grill chicken cutlets for 4 to 5 minutes per side until golden and cooked through. Remove to a cutting board and allow to rest for 2 minutes. Slice as desired.
04 - If using, lightly spread butter on the cut sides of each roll. Toast on the grill pan or skillet, cut sides down, until golden brown.
05 - Spread a generous spoonful of basil pesto onto the bottom half of each toasted roll.
06 - Arrange grilled chicken slices over the pesto. Top with mozzarella slices, tomato, and arugula or spinach leaves. Close each sandwich with the top half of the roll.
07 - Serve sandwiches warm or at room temperature.