Begin the batter 16-18 hours before you plan to bake the zucchini bread. This allows the sourdough starter to ferment the flour, enhancing its digestibility and nutrient availability.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
In another bowl, combine the sourdough starter, sugar, oil, eggs, and vanilla extract. Mix until well combined.
Add the wet ingredients to the dry ingredients and stir until just combined. Do not overmix.
Gently fold in the grated zucchini, and if using, add the chopped nuts.
Cover the mixture with a towel or plastic wrap and let it ferment at room temperature for 4-6 hours.
Transfer the batter to the fridge and let it continue fermenting overnight for an additional 12 hours.
The next day, preheat your oven to 350°F (175°C).
With a Danish dough whisk or a wooden spoon, give the zucchini sourdough dough a few good stirs.
Pour the batter into a loaf pan and bake for 55-65 minutes. Stick a long toothpick in the center and when it comes out clean it is ready.
Let the bread cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.
Slice and enjoy your sourdough zucchini bread!