Whisk together all marinade ingredients. Pour half over chicken breasts and let marinade for at least an hour.
Prep salad by chopping all veggies. Add lettuce to a big bowl, and top with olives, cucumber, tomatoes, onion, and pepperoncini.
Preheat grill to medium-high (around 400F) and spray with nonstick spray. Once hot, add chicken. Because we are using thin chicken breasts, they will take about 3 minutes per side. If using thicker breasts, they will take longer. Take chicken off once internal temperature of 165F.
Let chicken rest 10 minutes before cutting and putting on top of salad.
Sprinkle feta cheese on top, along with remainder of the dressing.
Enjoy!