01 - Butterfly the chicken breasts horizontally to create four thin fillets of even thickness.
02 - Whisk together olive oil, minced garlic, oregano, salt, and pepper. Coat chicken thoroughly and marinate at room temperature for 10 minutes.
03 - Combine mayonnaise, minced garlic, lemon juice, and salt in a small bowl. Mix until smooth and refrigerate until serving.
04 - Heat grill pan or skillet over medium-high heat. Cook chicken fillets 4–5 minutes per side until golden and internal temperature reaches 165°F. Add cheese slices during final minute if using.
05 - Lightly toast the cut sides of sandwich buns until golden brown.
06 - Spread garlic mayo on both bun halves. Layer lettuce, tomato slices, and grilled chicken on bottom halves. Cover with top bun.
07 - Serve sandwiches immediately while chicken is still warm.