Prepare the Oven and Pan: Preheat the oven to 350°F (175°C). Grease a 9 x 5-inch loaf pan and line it with parchment paper, leaving some overhang to help lift out the finished bread.
Grate the Zucchini: Wash and trim the zucchini, then grate it using the large holes of a box grater. Gently squeeze out excess liquid if the zucchini is particularly watery, but avoid drying it out completely.
Mix the Dry Ingredients: In a large bowl, whisk together the flour, cinnamon, baking soda, baking powder, salt, and nutmeg, if using. Break up any lumps so the leavening ingredients are evenly distributed.
Combine the Wet Ingredients: In a separate bowl, whisk the eggs, granulated sugar, brown sugar, vegetable oil, and vanilla extract until smooth. Stir in the grated zucchini until evenly incorporated.
Make the Batter: Add the dry ingredients to the zucchini mixture. Fold gently with a spatula until no dry patches remain. Avoid overmixing, as this can make the bread dense.
Fill the Loaf Pan: Pour the batter into the prepared pan and spread it evenly. Tap the pan gently on the counter to release any large air pockets.
Bake Until Set: Bake for 50–60 minutes, or until the top is golden and a toothpick inserted into the center comes out clean or with a few moist crumbs. If the top browns too quickly, loosely cover it with foil during the final 15 minutes.
Cool and Slice: Let the bread cool in the pan for 10–15 minutes before lifting it onto a wire rack. Allow it to cool completely before slicing for the cleanest cuts.