01 - Butterfly each chicken breast horizontally to create four thin, even fillets for quick and even cooking.
02 - Whisk together olive oil, minced garlic, oregano, salt, and black pepper in a shallow dish. Add chicken fillets and turn to coat thoroughly. Let marinate at room temperature for 10 minutes.
03 - While chicken marinates, combine mayonnaise, minced garlic clove, lemon juice, and salt in a small bowl. Mix until smooth and refrigerate until ready to use.
04 - Heat grill pan or skillet over medium-high heat. Cook marinated chicken fillets for 4–5 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 - Lightly butter buns if desired and toast cut-side down in a pan or toaster until golden and crisp.
06 - Spread a generous layer of garlic mayo on both halves of each toasted bun. Layer lettuce leaves on the bottom bun, followed by tomato slices, grilled chicken, and top bun. Serve immediately while warm.