1. Preheat the oven to 425 degrees . Line a rimmed baking sheet with aluminum foil and coat with cooking spray.
2. In a large bowl, lightly mash lentils with a fork. Add egg, 3 tablespoons Parmesan, mustard and pepper and stir to combine. Add turkey and bread crumbs to lentil mixture and knead gently until just combined.
3. Form turkey mixture into 12 meatballs, about 2 tablespoons each, and place on prepared baking sheet. Bake until no longer pink in the center, 14 to 16 minutes.
4. Meanwhile, combine pesto, vinegar and water in a small bowl. Brush bread with oil on one side and lay oil side down in a large skillet. Toast over medium heat, about 6 minutes. Divide bread among four plates, toasted side up. Top with meatballs, pesto mixture, arugula and, if desired, Parmesan.