Rinse lentils and cook according to package instructions. Drain and let cool.

In a large bowl, combine cooled lentils, drained tuna, bell pepper, cucumber, and red onion.

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

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

Add chopped parsley and mix lightly.

Chill for at least 30 minutes before serving for best flavor.