01 - Preheat grill pan or skillet over medium-high heat.
02 - Slice chicken breasts horizontally to create thin cutlets. Drizzle with olive oil and season both sides with salt and pepper.
03 - Place chicken cutlets on the hot grill or skillet. Cook for 4 to 5 minutes per side, until fully cooked and golden. Transfer to a plate and rest for 2 minutes. Slice if desired.
04 - If desired, spread butter on the cut sides of each roll. Toast on the grill or skillet, cut-side down, until golden and crisp.
05 - Spread a generous layer of pesto onto the bottom of each roll.
06 - Layer grilled chicken, fresh mozzarella slices, tomato, and arugula or spinach over the pesto. Cover with the top half of each roll.
07 - Serve sandwiches warm or at room temperature.