Preheat oven to 425 degrees Fahrenheit and line an 8.5 by 4.5 loaf pan (you can also use a 9 by 5 loaf pan) with parchment paper.
Wash 1 lb of zucchini and chop off the ends. Use a box grater (affiliate link) to shred the zucchini. Grab a sheet or two of paper towel. Add the shredded zucchini to the middle of the paper towel and wrap the zucchini up to form a ball. Squeeze the paper-towel ball of zucchini over the sink 2-3 times to wring most of the water out of the zucchini. Continue this process until you have 1 ½ cups of shredded zucchini.
To a bowl, add the flour, sugar, baking powder, baking soda, salt, cinnamon and allspice. Stir together with a fork until fluffy and combined. Add the zucchini and stir until the zucchini is spread throughout the dry mixture and thoroughly combined.
In a liquid measuring cup, measure out ½ cup of sourdough starter. Add the eggs, vegetable oil and greek yogurt. Stir well to combine.
Add the liquids to the dry ingredients. Mix together with a fork or spoon until just combined (over-mixing will result in tough zucchini bread and nobody wants that).
Pour the zucchini bread batter into the prepared loaf pan and bake for 10 minutes at 425 degrees. This helps ensure a nicely domed loaf of bread.
After 10 minutes reduce the temperature to 350 degree Fahrenheit. Bake for 45-55 minutes. Insert a toothpick or sharp knife into the center of the bread to check if it is ready. If it comes out clean with no streaks of batter, it is ready! If it has streaks of wet batter, bake it a little longer and check again.
Allow the zucchini loaf to cool for 10 minutes before removing from the loaf pans. Move the loaf to a cooling rack and cool completely before digging in. Enjoy!