Lay 1 chicken breast half, skin side up, between 2 large sheets of plastic wrap and, using the flat side of a meat mallet, pound out evenly until chicken is 1⁄2" thick. Remove from plastic wrap, place on a large plate, and repeat process with remaining chicken.
Heat a grill pan over medium heat until hot but not smoking. Sprinkle rosemary, red pepper flakes, and salt and pepper to taste over both sides of chicken. Working in batches if necessary, grill chicken, skin side down, until golden and crisp, about 5–6 minutes. Flip chicken over and continue to grill until golden, about 3 minutes. Transfer chicken, skin side up, to a clean plate. Top each breast with 3 overlapping slices of lemon and 2 lemon leaves lined up lengthwise. Carefully flip chicken, leaf side down, onto grill pan and cook until lemons are lightly browned, leaves are softened and slightly charred, and chicken is cooked through, 2–3 minutes more.
Arrange remaining lemon slices and leaves between 4 plates. Top each with a grilled chicken breast (and the lemons and leaves it was grilled with). Serve promptly, with extra lemon wedges, if you like.