To prepare the marinade, place the freshly-squeezed lemon juice and olive oil into a bowl.
Mince the garlic. Remove the leaves from the rosemary and finely chop. Add the garlic, rosemary and chili flakes to the marinade and whisk to combine. Make sure to taste the marinade and adjust it to suit your tastes. Set aside.
To prepare the chicken, evenly slice each breast in half horizontally. Place each half on the cutting board cut side up. Cover with plastic wrap and pound with a mallet to even out the thickness. Each half should be about 1/4" -inch thick when done. Season the chicken on both sides with salt and pepper to taste.
Place the chicken into a shallow dish and thoroughly coat with the marinade. Cover with plastic wrap and transfer to the refrigerator. Let marinate for approximately 1 hour.
When ready to cook, preheat your grill to high. Clean and then oil the grate with a paper towel that has been doused in oil.
Drain the chicken from the marinade and lightly pat dry with paper towels. Discard the marinade.
Place the chicken onto the grill diagonally (cut side up first). Grill each side for approximately two minutes or just until cooked through.
Transfer to a serving platter and cover with vented foil to rest for a few minutes.
Sprinkle with freshly chopped parsley and drizzle with a bit of extra-virgin olive oil, if desired. Serve.