Preheat oven to 350°F (175°C). Grease a standard 9×5-inch loaf pan or line with parchment paper.

Mix dry ingredients: In a medium bowl, whisk together flour, cinnamon, baking soda, salt, and nutmeg.

Mix wet ingredients: In a large bowl, beat together sugar and vegetable oil until well combined. Add eggs one at a time, mixing well after each. Stir in vanilla.

Combine: Add grated zucchini and stir to incorporate.

Combine wet and dry: Gradually mix dry ingredients into the wet mixture until just combined. Fold in chopped nuts, if using.

Prepare streusel topping: In a separate bowl, mix flour, brown sugar, granulated sugar, and cinnamon. Cut in cold butter until crumbly.

Assemble and bake: Pour batter into the loaf pan. Sprinkle streusel topping evenly over the top.

Bake for 60–70 minutes or until a toothpick inserted in the center comes out clean.

Cool in pan for 10 minutes before transferring to a wire rack to cool completely.