Add the oil, lime juice and zest, cilantro, garlic, cumin, salt and red pepper flakes to a gallon-sized freezer bag, seal, and shake. Add chicken and toss around.
Pound out the chicken to about 3/4 inch thick (or cut it in half lengthwise) to help with fast and even cooking. Grill for 4-6 minutes per side, closing the lid in between flipping.