Make the flax egg by combining 1 tbsp ground flaxseed with 2½ tbsp water in a small bowl. Set aside until the mixture becomes gel-like (around 10-15 minutes)
Preheat the oven to 350°F and line a muffin tin with 12 liners.
In a medium bowl, combine flour, salt, baking soda, baking powder, cinnamon, and nutmeg and set aside.
Shred the washed, unpeeled zucchini and measure out one cup, then pat dry with paper towels to remove some of the moisture. Set aside.
Whisk together the oil, milk, and vanilla extract. Whisk in the brown sugar and granulated sugar.
Add in the flax egg, then stir in the shredded zucchini.
Mix in the dry ingredients until combined, then fill the muffin tins ¾ of the way full.
Bake 12-15 minutes or until a toothpick inserted in the middle of a muffin comes out clean. Let cool in the tin for 5 minutes then transfer to a wire rack to cool completely.