In a small bowl, whisk together the white balsamic vinegar and Dijon mustard. Whisk in the olive oil, then stir in the scallions. Set the dressing aside. Whisk once more before using.
In a medium bowl, combine the lentils, cucumber, tomatoes, shallot, feta cheese, cilantro, and mint. Toss with the dressing. Season to taste with salt and pepper.
