In a medium saucepan, combine lentils and 4 cups of water. Bring to a boil, then reduce heat to low and simmer for about 20-25 minutes, or until lentils are tender but not mushy. Drain and set aside to cool.

In a small bowl, whisk together olive oil, lemon juice, cumin, salt, and black pepper.

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

Pour the dressing over the lentil mixture and toss gently to combine.

If using, sprinkle feta cheese on top before serving.

Chill in the refrigerator for at least 30 minutes to allow flavors to meld, then serve.