01 - Butterfly the chicken breasts horizontally to create four thin, even fillets for quick and even cooking.
02 - In a mixing bowl, whisk together olive oil, minced garlic, dried oregano, salt, and black pepper until well combined.
03 - Add the chicken fillets to the marinade, turning to coat thoroughly. Let stand at room temperature for 10 minutes to absorb flavors.
04 - While chicken marinates, combine mayonnaise, minced garlic, lemon juice, and salt in a small bowl. Mix until smooth and refrigerate until ready to use.
05 - Heat a grill pan or skillet over medium-high heat. Cook chicken fillets for 4–5 minutes per side until golden brown and cooked through, reaching an internal temperature of 165°F. Add cheese slices during the final minute to melt, if using.
06 - Lightly toast the cut sides of the sandwich buns until golden and crisp.
07 - 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.