Rinse the quinoa thoroughly under cold water to remove any bitterness. Drain well.
In a medium-sized saucepan, bring the water or vegetable broth to a boil. Add the quinoa and reduce the heat to low. Cover and simmer for about 15-20 minutes, or until the quinoa is tender and the liquid is absorbed. Remove from heat and let it cool.
In a large mixing bowl, combine the cooked quinoa, cherry tomatoes, cucumber, bell pepper, red onion, parsley, mint, and Kalamata olives. Toss gently to combine.
In a small bowl, whisk together the olive oil, lemon juice, minced garlic, Dijon mustard, salt, and pepper to make the dressing.
Pour the dressing over the quinoa salad and toss well to coat all the ingredients.
If using feta cheese, sprinkle it over the salad and gently toss.
Taste and adjust the seasonings if needed.
Cover the bowl and refrigerate the quinoa salad for at least 30 minutes to allow the flavors to meld.
Serve chilled as a side dish or as a light and refreshing main course.