In a large bowl, combine lemon juice, olive oil, Italian seasoning, salt, pepper, garlic, red pepper flakes, mayonnaise, and onion powder. Mix well.
Add 2-3 Rosemary sprigs to your lemon marinade.
Add your chicken breasts to your bowl, making sure that each is thoroughly coated in marinade.
Marinade for at least 20 minutes covered in the refrigerator.
Preheat your grill to medium-high heat.
Grill chicken breasts until internal temperature reaches 165 degrees, about 7 minutes on each side.
Serve warm with remaining rosemary sprigs, parsley, and lemon.