Let's get everything ready to make the bread by preheating the oven to 350°F (177°C). Lightly grease a 9 × 5-inch loaf pan and line it with parchment paper, leaving enough overhang to easily lift out the bread. Measure all ingredients, finely chop the walnuts, and bring the eggs and sour cream to room temperature. Wash and dry the zucchini, then trim off the ends. Using the large holes of a box grater, grate the zucchini until you have 1 cup, lightly packed. Do not peel or squeeze the zucchini, the natural moisture helps keep the bread tender and moist.
In a large mixing bowl, whisk together the eggs, granulated sugar, and dark brown sugar until smooth and well combined. Add the vegetable oil, sour cream, and vanilla extract and whisk again until incorporated. Fold the grated zucchini into the wet mixture with a rubber spatula until evenly distributed.
In a separate medium bowl, whisk together the flour, cinnamon, nutmeg, salt, baking soda, and baking powder until evenly combined.
Add the dry ingredients to the wet ingredients and gently fold with a rubber spatula until just combined and no large streaks of flour remain. Avoid over-mixing, which can make the zucchini bread dense or tough. Gently fold the chopped walnuts into the batter just until evenly distributed.
Transfer the batter to the prepared loaf pan and use the spatula to spread it evenly and smooth the top. Bake at 350°F (177°C) for 50-60 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean or with a few moist crumbs. If the top begins browning too quickly before the center is done, loosely tent the loaf with aluminum foil.
Allow the zucchini bread to cool in the pan for 15 minutes. Use the parchment overhang to carefully lift it from the pan, then transfer it to a wire rack to cool completely before slicing. Enjoy!