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. Coat chicken thoroughly in the marinade and let sit at room temperature for 10 minutes.
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 a grill pan or skillet over medium-high heat. Grill chicken fillets for 4 to 5 minutes per side until cooked through with an internal temperature of 165°F and golden brown. Add cheese slices during the final minute to melt if desired.
05 - Lightly toast the split buns or ciabatta rolls until golden and crisp.
06 - Spread a generous layer of garlic mayo on both halves of each toasted bun. Layer lettuce leaves, tomato slices, and grilled chicken on the bottom halves. Close with the top bun and serve immediately.