Preheat oven to 350 degrees F (175 degrees C). Grease 12 muffin cups, or line with paper muffin liners.
Combine flour, sugar, baking soda, cinnamon, and salt in a bowl. Mix egg, oil, milk, lemon juice, and vanilla extract in another bowl; stir into dry ingredients until just moistened. Fold in zucchini and walnuts. Fill prepared muffin cups 2/3 full.
Bake in preheated oven until a toothpick inserted into the center of a muffin comes out clean, 20 to 25 minutes.