Marinate the chicken breasts in a gallon-sized zip bag with 2 tablespoons olive oil, lemon zest, lemon juice, garlic, oregano, salt, and pepper. Marinate for a minimum of 20 minutes.
In a large skillet, heat the remaining tablespoon of olive oil over medium-high heat, add the chicken breasts into the skillet, and cook for 7 minutes, flip and continue to cook for another 5-7 minutes until the internal temperature has reached 165. Discard marinade. Remove chicken from pan and wait 5 minutes before slicing.
Meanwhile, make a bed of lettuce in 4 individual bowls or one large salad bowl.
When the chicken is ready, top the salad with sliced chicken, tomatoes, cucumber, olives, red onion, feta cheese, and tzatziki sauce (if using).