Preheat the oven to 350 degrees F.
Heat 1 tablespoon oil in a small skillet over medium-low heat until hot but not smoking. Add the chopped onion and saute until softened and brown, about 6 minutes. Add the parsley and cook for another minute. Set aside.
In a large mixing bowl, combine the turkey with the breadcrumbs, egg, 1/4 cup of the ketchup, cooked onion mixture, salt and pepper. Mix until fully combined. Form into a loaf and transfer to a 9-by-5-inch loaf pan. Using a pastry brush or the back of a spoon, brush the remaining 3 tablespoons of ketchup over the loaf.
Evenly scatter the sliced onions over the loaf, drizzle with the remaining tablespoon of oil and sprinkle with salt and pepper. Bake until the meatloaf is cooked through, 55 to 60 minutes.