In a medium saucepan, combine the lentils and 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 let cool.

In a large bowl, combine the diced cucumber, cherry tomatoes, red onion, bell pepper, parsley, and feta cheese (if using).

In a small bowl, whisk together the olive oil, red wine vinegar, dried oregano, salt, and pepper.

Add the cooled lentils to the vegetable mixture and pour the dressing over the top. Toss gently to combine.

Chill the salad in the refrigerator for at least 30 minutes before serving to allow the flavors to meld.