Rinse the quinoa under cold water in a fine-mesh sieve to remove any bitterness. Drain well.
In a medium saucepan, combine the rinsed quinoa and water (or vegetable broth). Bring to a boil over medium-high heat.
Once boiling, reduce the heat to low, cover the saucepan, and let it simmer for about 15 minutes or until the quinoa is fluffy and all the liquid is absorbed. Remove from heat and let it sit, covered, for an additional 5 minutes.
In a large mixing bowl, add the diced cucumber, cherry tomatoes, bell pepper, red onion, feta cheese, and parsley.
Fluff the cooked quinoa with a fork and allow it to cool slightly before adding it to the mixing bowl with the vegetables.
In a small bowl, whisk together the olive oil, lemon juice, salt, and pepper. Pour this dressing over the quinoa and vegetables.
Toss everything together until well combined. Adjust seasoning with more salt and pepper if needed.
Serve the salad immediately or chill in the refrigerator for about 30 minutes before serving for a refreshing taste.