Heat the oven to 350°F (175°C). Grease a 9 x 5-inch loaf pan and line it with parchment paper, leaving enough overhang to lift the baked loaf out easily.

Peel and core the apple, then shred it using the large holes of a box grater. Lightly squeeze the shredded apple once or twice to remove excess liquid, but do not press it completely dry. You want the apple moist but not dripping.

In a medium bowl, whisk together the flour, baking soda, baking powder, 1/2 teaspoon of the cinnamon, ginger, nutmeg, and salt. This distributes the leaveners and spices evenly.

In a large bowl, whisk together the sourdough starter, pumpkin puree, brown sugar, oil, eggs, and vanilla until smooth. The mixture should be thick and evenly blended.

Fold the shredded apple into the wet mixture until evenly distributed. If the apple has released a large amount of juice, do not add that excess liquid to the batter.

Add the dry ingredients to the wet mixture: Fold gently with a rubber spatula just until no dry streaks remain. The batter should be thick and spoonable. Do not overmix, or the loaf may bake up dense.

Transfer the batter to the prepared loaf pan and smooth the top. If using the optional turbinado sugar, sprinkle it evenly over the surface.

Bake at 350°F (175°C) for 55 to 65 minutes. The loaf should be domed and golden brown, and a toothpick inserted into the center should come out with a few moist crumbs but no wet batter.

If the top becomes deeply browned before the center is done, loosely cover the loaf with aluminum foil during the final 15 minutes of baking.

Let the bread cool in the pan for 15 minutes, then lift it out using the parchment and transfer it to a wire rack. Let it cool for at least another 30 minutes before slicing. The crumb will firm as it cools.