Preheat the oven to 325°F (162°C), and coat two 8 x 4-inch (20cm x 10cm) pans with nonstick spray.
In a stand mixer, beat the eggs, oil, sugar and vanilla extract on medium speed with the paddle attachment.
Add the salt, baking soda, baking powder, cinnamon and nutmeg and mix until thoroughly incorporated. Then mix in the flour in thirds. Scrape down the bowl, as needed, to ensure the ingredients are thoroughly combined.
Stir in the zucchini and walnuts.
Divide the batter evenly between the two pans, and bake for 1 hour or until a toothpick inserted into the center comes out clean.
After placing the bread in the oven, make the crumb topping by combining all the ingredients in a bowl and mixing with hands, until large crumbles form. Once the bread has baked for 15 minutes, divide the crumbs evenly and scatter them over the top of each pan of bread. Continue baking.
Allow the bread to rest on the counter for 20 minutes, before removing it from the pan to cool completely on a wire rack.
Notes
For more green coloring in the bread, grate the zucchini on a larger-sized setting.