Step 1: Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease well with butter.
Step 2: In a medium bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, and nutmeg until well combined.
Step 3: In a large bowl, combine the sourdough discard, oil, brown sugar, egg, and vanilla extract. Whisk until the mixture is smooth and the sugar has mostly dissolved.
Step 4: Gradually add the dry ingredients to the wet ingredients, stirring gently with a spatula just until combined. Do not over-mix.
Step 5: Fold in the grated zucchini until evenly distributed throughout the batter.
Step 6: Scoop the batter into the prepared muffin tin, filling each cup about three-quarters full.
Step 7: Bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Step 8: Let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.