Bring quinoa and water to a boil over medium heat. Reduce to a simmer and cook for 15-20 minutes or until water is absorbed in quinoa. Fluff with a fork and cool for 5 minutes.
Add the oil and vinegar and let quinoa come to room temperature.
Add onion, tomatoes, pepper, olives, cilantro, and salt and pepper. Stir gently. Add feta cheese and gently combine. Chill for 2 hours to let flavors combine. Squeeze lemon juice over salad right before serving to brighten flavors.