Heat oven to 400° F. In a bowl, combine the turkey, onion, spinach, parsley, bread crumbs, mustard, egg white, and ½ 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 enough water to cover and bring to a boil. Reduce heat and simmer until tender, 15 to 18 minutes. Drain the potatoes and return them to the pot. Mash with the buttermilk, oil, ½ teaspoon salt, and ¼ teaspoon pepper.
Serve the meat loaf with the potatoes and the marinara sauce, if using.