Preheat your oven to 325°F and grease a 9x5-inch loaf pan.
In a large bowl, whisk together the flour, sugar, cinnamon, baking powder, baking soda, and salt. Set aside.
In a medium bowl, mix together the grated zucchini, oil, sourdough discard, eggs, and vanilla extract.
Pour the zucchini mixture into the dry ingredients and fold until JUST combined. Fold in the walnuts.
Pour the batter into the loaf pan and bake for 70-80 minutes, or until a toothpick inserted into the center comes out clean. I like to cover with a sheet of aluminum foil halfway through to prevent the top from browning too much.
Let the bread cool in the pan for 15 minutes before transferring to a wire rack to cool completely.
Nutrition information is calculated using an ingredient database and should be considered an estimate.