Cut each chicken breast in half to make 6-8 thinner chicken cutlets.
In a large bowl, add chicken, lemon juice + zest, rosemary, garlic, olive oil, salt and pepper. Stir with tongs to coat evenly, cover with plastic wrap and refrigerate for at least 30 minutes or up to 24 hours.
Preheat grill on medium heat around 500 degrees F and spray with cooking spray. Add chicken and lemon slices in single layer to the grill. Discard the marinade.
Grill for 5 – 6 minutes covered, turn chicken over and place a lemon slice grilled side up on top. Cover and grill for another 5 – 6 minutes.
Remove chicken from the grill onto a platter, cover and let rest for 5 minutes. Serve hot, with pita, veggie kebabs and (Greek yogurt tzatziki), if desired