Place each chicken breast in a Ziplock or between sheets of plastic wrap.
Using a meat mallet or rolling pin, carefully thin the chicken breasts into a consistent thickness of roughly 1/4 inch.
Combine the garlic, rosemary, salt, and pepper.
Gently rub the mixture on both sides of the pounded chicken breasts, then combine in a bowl with the fresh lemon juice and oil.
Refrigerate for as little as 30 minutes (or until your grill has come to temperature), but no longer than 4 hours.
Stabilize your grill at 550°F. Slightly oil wipe your grill grates.
Place the chicken on the grill (presentation side down first).
Grill undisturbed for 1.5 minutes, rotate and place on a new spot on the grill surface to assist in a better sear. Cook for 1 more minute.
Flip and continue to grill until the breasts are cooked through and have reached an internal temperature of 160°F.
Take the grilled chicken off the grill and stack on a cutting board, allowing it to rest and carry over to 165°F internal temperature (roughly 7 minutes).
Slice each breast 3 times across the grain on a slight bias. Serve immediately and enjoy!