Combine the turkey, onion, bread crumbs, egg, Parmesan, 2 tablespoons of the mustard, the parsley, red peppers, 3/4 teaspoon salt and 1/2 teaspoon pepper in a large bowl.
Shape the meat into an 8-inch loaf and place in a baking dish (not a loaf pan). Bake until no trace of pink remains, about 45 minutes. Transfer to a cutting board and let rest 15 minutes before slicing.
Whisk together the remaining mustard, vinegar, oil, 1/4 teaspoon salt and 1/4 teaspoon pepper in a small bowl.
Divide the greens among individual plates and drizzle the vinaigrette over the top. Serve with meatloaf.