01 - Butterfly the chicken breasts to create four thin fillets of even thickness.
02 - Combine olive oil, minced garlic, oregano, salt, and pepper in a bowl. Add chicken, turning to coat thoroughly. Let marinate at room temperature for 10 minutes.
03 - While chicken marinates, whisk together mayonnaise, minced garlic, lemon juice, and salt in a small bowl until smooth. Set aside.
04 - Heat a grill pan or skillet over medium-high heat. Cook chicken fillets for 4 to 5 minutes per side until golden and cooked through to an internal temperature of 165°F. Add cheese slices during the final minute if using, to melt.
05 - Spread garlic mayonnaise on both halves of each toasted bun. Layer lettuce, tomato slices, and grilled chicken on the bottom halves. Close with the top bun.
06 - Serve sandwiches immediately while chicken is warm.