Place the lentil in a pot and cover with water by three inches. Bring to a boil, then reduce the heat to a simmer and cook until the lentils are “al denteâ€. Drain, and cool the lentils to room temperature. Place the lentils, onions, celery, cucumber, cherry tomatoes in a bowl and toss. Place the dressing ingredients into a blender and pulse until blended. Pour the dressing over the lentil mix and toss well. Add the arugula and toss gently. Place in a large bowls and enjoy!