Preheat your oven to 350°F (175°C).
Line a 12-cup muffin tin with paper liners or lightly grease with oil or butter.
Prep the veggies. Grate zucchini and carrot on the fine side of a box grater. Place grated zucchini in a clean towel and squeeze out excess moisture. Finely chop spinach if using.
Measure 1 cup total.
In a large bowl, whisk together the whole wheat flour, all-purpose flour, baking powder, baking soda, cinnamon, and salt.
In a separate bowl, whisk the mashed banana, applesauce, milk, oil, egg, honey or maple syrup (if using), and vanilla until smooth.
Pour the wet ingredients into the dry. Stir gently with a spatula until just combined. Do not overmix—a few small lumps are fine.
Fold in the grated vegetables and any optional add-ins. The batter will be thick but scoopable.
Divide the batter evenly among the muffin cups, filling each about 3/4 full.
For mini muffins, fill mini wells nearly to the top.
Bake regular muffins for 16–20 minutes, or mini muffins for 10–13 minutes, until the tops spring back and a toothpick comes out clean or with a few moist crumbs.
Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. This helps the muffins set and stay tender.