Preheat your oven to 350°F and line a muffin tin with paper liners or lightly grease it.
In a large bowl, whisk together the whole wheat flour, baking powder, baking soda, salt, and pumpkin pie spice.
In a small bowl, combine the soy milk and lemon juice. Let it sit for 5 minutes to create a vegan “buttermilk.”
In another bowl, mash the banana until smooth. Add the shredded carrots, shredded zucchini, maple syrup, avocado oil, and vanilla extract. Mix until well combined.
Pour the soy milk mixture into the banana-carrot mixture and stir to combine.
Gradually fold the wet ingredients into the dry ingredients until just combined. If using, fold in the walnuts.
Divide the batter evenly among the muffin cups, filling each about ¾ full.
Bake for 18–22 minutes, or until a toothpick inserted in the center comes out clean.
Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.