In a large bowl, toss together the lettuce, onion, olives, tomatoes, parsley, and feta cheese
Combine the oil, vinegar, garlic, zest, and juice of the lemon in a mason jar and shake until combined
Put the chicken breasts in a shallow dish and use enough of the dressing to cover the chicken (save the remaining dressing for the salad)
Allow the chicken to marinate in the dressing for 30 minutes
Heat grill to medium high (375-450 degrees) and cook chicken breasts 5-6 minutes per side until the internal temperature reaches 150-155 degrees
Let chicken breasts rest 10 minutes
While the chicken is resting, add remaining dressing to the salad and toss
Slice the chicken and lay across the top of the salad or cut into bite size pieces and toss with the salad