Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper.

In a large bowl, combine the grated zucchini, granulated sugar, brown sugar, vegetable oil, eggs, and vanilla extract. Mix until well combined.

In another bowl, whisk together the flour, baking soda, baking powder, cinnamon, salt, and nutmeg. Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Do not overmix.

Pour the batter into the prepared loaf pan and spread it evenly.

In a separate bowl, mix together the flour, rolled oats, brown sugar, and cinnamon for the streusel topping. Drizzle in the melted butter and mix until crumbly. Sprinkle the streusel evenly over the top of the zucchini bread batter.

Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean. If the top browns too quickly, cover it loosely with aluminum foil.

Once baked, remove the loaf from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a wire rack to cool completely before slicing.