Heat oven to 400°F. Line a 12-count muffin tin with paper liners or spray with cooking spray.
Whisk together the 1 1/4 cups flour, 1 cup oats, 1/2 cup brown sugar, 1/2 teaspoon baking soda, 1/2 teaspoon baking powder, 1/2 teaspoon nutmeg, and 1/2 teaspoon salt in a large mixing bowl. Add 1/3 cup almond milk, 1/4 cup olive oil, 1 large egg, the shredded carrots, shedded apple, and mashed banana and stir until blended with a rubber spatula or a wooden spoon. Batter will be very thick.
Divide batter evenly between cups filling to the top. Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Allow the muffins to cool for 5 minutes in the muffin pan, then transfer to a wire cooling rack to continue cooling.
While the muffins cool, make the lemon glaze: Whisk together the 1 1/2 teaspoons lemon zest, 1 cup powdered sugar, and 2 tablespoons lemon juice until smooth.
Once muffins are cool, drizzle the tops with the lemon glaze and serve!