In a small bowl, combine the onions, cilantro, parsley, red wine vinegar, and olive oil.
Remove the backbone from the turkey breast, keeping both sides intact. Cut the breast into 2-inch steaks. Place the turkey cutlets into the marinade, cover, and marinate overnight in the refrigerator.
Heat a grill.
Grill the turkey cutlets until cooked and still juicy, about 5 to 6 minutes per side.