01 - Butterfly the chicken breasts horizontally to create four thin, even fillets for even cooking.
02 - Whisk together olive oil, minced garlic, oregano, salt, and pepper in a bowl. Add chicken fillets, turning thoroughly to coat. Let marinate 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. Cook chicken fillets for 4-5 minutes per side until golden and cooked through to an internal temperature of 165°F. Add cheese slices during the final minute to melt, if desired.
05 - Lightly toast the split buns in a toaster or on the grill pan for 1-2 minutes until golden and crispy.
06 - Spread a generous layer of garlic mayo on both halves of each toasted bun. Place lettuce leaves on the bottom half, followed by tomato slices and grilled chicken. Top with the remaining bun half.
07 - Serve sandwiches immediately while chicken is warm and bun remains crisp.