Whisk together the lemon juice, vinegar, oregano, garlic, salt and pepper (to taste) in a small bowl. Slowly whisk in the oil until it emulsifies. Let sit at room temperature while you prepare the salad to allow the flavors to marry.
Rinse the quinoa in a strainer until the water runs clear. Place quinoa, 2 cups chicken stock, 1 teaspoon sea salt and ¼ teaspoon pepper in a saucepan, bring to boil and cook until the liquid is absorbed and the quinoa is tender, about 15 minutes.
Transfer to a bowl, cover and chill for a couple of hours or overnight.
Remove quinoa from refrigerator and add tomatoes, red onion, olives, cucumber, peppers and dressing. Stir to combine. Transfer to serving bowl and sprinkle with feta.