Preheat the oven to 360 F. Lightly coat (2 or 3) 7 x 3 x 2 mini loaf pans or (1) 9 x 5-inch loaf pan with butter. Note: Using 2 mini loaf pans instead of 3 will you get you slightly taller loaves.

Using a box grater, grate the zucchini on an angle on the side with largest holes. No need to drain out the excess water. Set aside.

Add the banana, sugar, and vanilla to a large mixing bowl. Cream with a hand held mixer or stand mixer fitted with the paddle attachment, about 30 seconds to 1 minute (some small lumps of banana are okay).

Add the eggs, one at a time until fully incorporated. Add the sourdough starter.

Sift the flour, cinnamon, nutmeg, baking soda, and salt together in a separate bowl. Working in batches, add this to the banana mixture.

Add the oil and milk and mix until just combined. Do not over do it; the banana bread will be tough. The texture should be thick and pourable, but not runny. Add more flour if needed. Fold in the grated zucchini.

Pour the batter into the prepared pan(s). For 2 mini loaf pans, fill about ¾’s full. For 3 mini loaf pans, fill about ⅔’s full (these loaves will not be as high). Place onto a baking sheet and transfer to the oven.

Bake for 40-45 minutes for the 2 mini loaf pans (about 30-35 minutes if using 3 mini pans) or 60-65 minutes or more for the standard 9 x 5-inch pan. Cover with foil if the loaf browns too quickly.

Cool in the pan for 20 minutes, and then transfer to a wire rack to cool completely.