01 - Butterfly the chicken breasts horizontally to create four thin, even fillets for quick and even cooking.
02 - Whisk together olive oil, minced garlic, oregano, salt, and pepper in a bowl. Add chicken fillets and turn to coat thoroughly. Let stand at room temperature for 10 minutes to absorb flavors.
03 - While chicken marinates, combine mayonnaise, minced garlic, lemon juice, and salt in a small bowl. Mix until smooth and refrigerate until ready to use.
04 - Heat grill pan or skillet over medium-high heat. Cook marinated chicken fillets for 4 to 5 minutes per side until golden brown and cooked through, reaching an internal temperature of 165°F. If adding cheese, place slices on chicken during the final minute to melt.
05 - Lightly toast split sandwich buns or ciabatta rolls until golden and crispy.
06 - Spread generous amount of garlic mayonnaise on both halves of each toasted bun. Layer lettuce leaves, tomato slices, and grilled chicken on bottom halves. Cover with top bun and press gently.
07 - Serve sandwiches immediately while chicken is warm and cheese is melted.