Heat oven to 400° F. In a bowl, combine the turkey, onion, spinach, parsley, bread crumbs, mustard, egg white, and 1/2 teaspoon each salt and pepper.
Transfer the mixture to a baking sheet and form it into a 10-inch loaf. Spread with the ketchup. Bake until cooked through, 45 to 50 minutes.
Meanwhile, place the potatoes in a large pot of cold water. Simmer until tender, 15 to 18 minutes. Drain the potatoes and mash in the pot with the buttermilk, oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper.
Serve the meat loaf with the marinara sauce (if using) and potatoes.