Heat oven to 400° F.
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. (See Tip, below.)
Bake until no trace of pink remains (internal temperature should be 165° F), about 45 minutes. Transfer to a cutting board and let rest 15 minutes before slicing.
Whisk together the remaining mustard, the 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 the sliced meat loaf.
Tip: A loaf pan isn't the best choice for baking meat loaf. Instead use a large pan with 1 1/2- to 2-inch sides, such as a metal, glass, or ceramic 7-by-11-inch baking dish, which will allow the heat to circulate over the meat, cooking it evenly.