Rinse the lentils under cold water and drain. In a medium pot, combine lentils and water. Bring to a boil, then reduce heat and simmer for 15-20 minutes until tender but not mushy. Drain and let cool.

In a large bowl, combine the cooled lentils, diced cucumber, halved cherry tomatoes, chopped red onion, and fresh parsley.

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

Pour the dressing over the lentil mixture and toss gently to combine, ensuring all ingredients are evenly coated.

If using, sprinkle crumbled feta cheese over the salad and gently mix it in.

Let the salad sit for at least 10 minutes before serving to allow the flavors to meld. Serve chilled or at room temperature.