In a mixing bowl, combine olive oil, red wine vinegar, minced garlic, oregano, salt, and black pepper. Add the flank steak and coat well. Marinate for at least 15 minutes.

In a saucepan, bring 2 cups of water to a boil. Add quinoa, reduce heat to low, cover, and simmer for 15 minutes or until water is absorbed. Fluff with a fork.

Preheat the grill to medium-high heat. Grill the marinated steak for about 4-5 minutes per side for medium-rare. Remove from grill and let rest for 5 minutes before slicing.

In each bowl, add a serving of quinoa, top with sliced steak, cherry tomatoes, cucumber, red onion, feta cheese, olives, and parsley. Drizzle with additional olive oil if desired.