Place each chicken breast between sheets of plastic film. With a meat mallet, carefully pound to a uniform thickness of about ¼ in. (6 mm).
Combine the garlic, rosemary, salt, and pepper. Lightly rub the flattened chicken breasts on both sides with the mixture. Sprinkle both sides with the lemon juice, then with the olive oil. Let marinate 2–4 hours in the refr