Prep the oven and pan: Preheat your oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners or lightly grease it.
Grate and drain the zucchini: Use the large holes of a box grater.
Gather the zucchini in a clean kitchen towel and gently squeeze to remove excess water. Don’t wring it dry—just reduce the excess.
Mix dry ingredients: In a large bowl, whisk flour, granulated sugar, brown sugar, baking soda, baking powder, salt, cinnamon, and nutmeg until combined and lump-free.
Whisk wet ingredients: In a separate bowl or large measuring cup, whisk the egg, oil, milk, and vanilla until smooth.
Combine gently: Pour the wet ingredients into the dry. Stir with a spatula until just a few streaks of flour remain.
Do not overmix.
Fold in zucchini and add-ins: Add the grated zucchini and any optional mix-ins. Fold just until evenly distributed and no dry flour pockets remain.
Fill the muffin cups: Divide the batter evenly among the 12 cups, about 3/4 full. Sprinkle coarse sugar on top if using.
Bake: Bake for 18–22 minutes, or until the tops spring back and a toothpick comes out with a few moist crumbs.
Cool: Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to finish cooling.
Enjoy warm or at room temperature.