Position a rack in the center of your oven and preheat to 350°F. Grease a 9x5 inch loaf pan with butter or nonstick spray, then line with parchment paper leaving a 2-inch overhang on the long sides for easy removal.

In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt until evenly combined. Set aside.

In a large bowl, whisk the oil, brown sugar, and granulated sugar until smooth and no brown sugar lumps remain, about 1 minute. Add the eggs one at a time, whisking well after each, then stir in the vanilla.

Add the grated zucchini and diced apple to the wet ingredients and stir to combine. The mixture will look slightly separated - this is normal.

Pour the dry ingredients over the wet and fold together with a rubber spatula just until no flour streaks remain. Do not overmix; a few small lumps are fine. The batter will be thick and chunky.

Scrape the batter into the prepared pan and smooth the top. Bake for 50 to 60 minutes, until a toothpick inserted into the center comes out clean or with just a few moist crumbs. The top should be deep golden and slightly cracked.

Cool in the pan on a wire rack for 15 minutes, then use the parchment overhang to lift the bread out and cool completely on the rack, about 1 hour. Slice with a serrated knife.