Rinse the quinoa under cold water. In the medium saucepan, combine the quinoa and water or vegetable broth. Bring to a boil, reduce heat to low, cover, and let simmer for about 15 minutes, or until the liquid is absorbed and the quinoa is tender.
While the quinoa is cooking, prepare the vegetables. Halve the cherry tomatoes, dice the cucumber, thinly slice the red onion, and slice the olives.
In a large bowl, combine the cooked quinoa, tomatoes, cucumber, red onion, and olives.
Drizzle the olive oil and lemon juice over the mixture, and season with salt and pepper to taste. Toss to combine.
Sprinkle with feta cheese (if using) and fresh parsley for garnish.
Serve warm or chilled for a quick, healthy, and flavorful meal.