Preheat your oven to 350°F and lightly grease a 9x5-inch loaf pan.
Grate the zucchini using the large holes of a box grater. Place it in a colander, gently squeeze out the excess liquid, and let it sit while you prep the rest of the ingredients.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set this aside.
In a separate bowl, combine the sugars, eggs, sourdough discard, vanilla, and oil. Stir until smooth and well blended.
Add the dry ingredients to the wet mixture and stir just until everything is combined—be careful not to over mix. Then fold in the grated zucchini.
Pour the batter into your prepared loaf pan and smooth out the top. Sprinkle on the walnuts and sugar for the topping. Bake for 55–60 minutes, or until a toothpick inserted into the center comes out clean.
Cool the bread in the pan for 10–15 minutes, then transfer to a wire rack to cool completely. Slice and enjoy!
