Cook the Quinoa
Rinse the quinoa under cold water in a fine mesh colander to remove any bitterness.
In a medium saucepan, combine the quinoa and water (or vegetable broth). Bring to a boil, then reduce to a simmer. Cover and cook for 12-15 minutes, or until the quinoa is tender and the water is absorbed.
Remove from heat and let the quinoa sit, covered, for 5 minutes. Fluff with a fork, then set aside to cool.
Prepare the Salad Ingredients
While the quinoa cools, chop the tomatoes, cucumber, and red onion.
Slice the olives and crumble the feta cheese.
Make the Dressing
In a small bowl, whisk together the olive oil, lemon juice, dried oregano, salt, and pepper. Taste and adjust seasoning as needed.
Assemble the Salad
In a large mixing bowl, combine the cooked and cooled quinoa with the chopped vegetables, olives, and feta cheese.
Pour the dressing over the salad and toss gently to combine.
Chill and Serve
Chill the salad in the refrigerator for at least 30 minutes to allow the flavors to meld.
Garnish with fresh parsley, if desired, and serve cold.