01 - Butterfly the chicken breasts horizontally to create four thin, even fillets for quick and uniform cooking.
02 - Whisk together olive oil, minced garlic, dried oregano, salt, and black pepper in a bowl. Add chicken fillets, turning to coat thoroughly. Let stand at room temperature for 10 minutes to absorb flavors.
03 - Combine mayonnaise, minced garlic clove, lemon juice, and salt in a small bowl. Mix until smooth and homogeneous. Refrigerate until ready to use.
04 - Heat grill pan or skillet over medium-high heat. Place marinated chicken fillets in the pan and cook for 4 to 5 minutes per side until golden brown and cooked through to an internal temperature of 165°F. If using cheese, place slices on chicken during the final minute to melt.
05 - Split sandwich buns and toast cut sides until lightly golden and crispy.
06 - Spread a generous layer of garlic mayonnaise on both halves of each toasted bun. Place lettuce leaves on the bottom half, followed by tomato slices and grilled chicken. Close with the top bun.
07 - Serve sandwiches immediately while chicken is warm and bread retains its crunch.