Cook the quinoa: In a medium saucepan, bring 2 cups of water to a boil. Add the rinsed quinoa, reduce heat to low, cover, and simmer for 15 minutes or until the water is absorbed. Remove from heat and let it sit covered for 5 minutes. Fluff with a fork and let it cool.

Prepare the vegetables: While the quinoa is cooking, dice the cucumber, red bell pepper, and halve the cherry tomatoes. Finely chop the red onion, parsley, and mint.

Make the dressing: In a small bowl, whisk together olive oil, lemon juice, Dijon mustard, minced garlic, salt, and black pepper.

Combine ingredients: In a large bowl, mix the cooled quinoa with the diced vegetables, chopped herbs, and crumbled feta cheese (if using).

Add the dressing: Pour the dressing over the quinoa mixture and toss gently to combine all ingredients evenly.

Chill and serve: Cover the salad and refrigerate for at least 30 minutes to allow flavors to meld. Serve chilled.