Preheat oven to 350°F (175°C) and grease a 9×5-inch loaf pan.
Grate the zucchini and lightly squeeze out excess moisture, keeping it slightly damp.
Whisk together the flour, sugar, baking soda, baking powder, cinnamon, salt, and nutmeg.
In another bowl, whisk the eggs, oil, sour cream, and vanilla until smooth.
Stir the zucchini into the wet mixture.
Add the dry ingredients to the wet mixture and gently mix until just combined.
Mix the cinnamon and sugar together in a small bowl.
Pour half the batter into the loaf pan, sprinkle a thick layer of cinnamon sugar, add the remaining batter, and top with the rest of the cinnamon sugar.
Bake for 50–60 minutes, or until a toothpick comes out clean. Tent with foil if the top browns too quickly.
Cool in the pan for 10 minutes, then transfer to a rack to cool completely.