Whisk together olive oil, lemon juice, minced garlic, salt and pepper. Then add the diced onion.
Transfer marinade to a gallon sized plastic bag or container and add the chicken. Place in the fridge and marinate overnight.
Place the chicken on a preheated grill and sprinkle with pepper. Cook for about 20 minutes or until cooked through, with no pink in the center. Grill additional lemon slices, for garnish.