Preheat the oven to 375°F (190°C). Line 12 standard muffin cups with paper liners, or grease the cups if you don't want to use liners.
Whisk together the flour, sugar, baking powder, baking soda, salt, and lemon zest in a large bowl. Make a well in the center.
In a separate bowl, stir together the butter, milk, eggs, and lemon juice. Pour into the well in the flour mixture. Mix just until combined or a few streaks of flour remain.
Stir in the zucchini and nuts.
Divide the batter evenly among the prepared muffin cups, using about 3 tablespoons of batter per cup to fill them 3/4 full. If you like, sprinkle a pinch of coarse sugar over the top of each muffin.
Bake 20 to 25 minutes, or until the muffins are golden brown and a pick inserted into the center comes out clean.
Cool the muffins in the pan on a wire rack for 10 minutes. Then transfer the muffins from the pan directly onto a wire rack to cool completely.
Notes
*Weight measurements for zucchini can vary from 120g to 150g, depending on the moisture content. Place a paper towel over the shredded zucchini to absorb some of the moisture before mixing it into the batter.
Store the muffins in an airtight container at room temperature for up to 3 days, or freeze up to 3 months.