Prep the pan and oven: Heat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or lightly coat with nonstick spray.
Whisk the dry ingredients: In a large bowl, whisk the white whole wheat flour, all-purpose flour, baking powder, baking soda, salt, cinnamon, nutmeg, and sugar until well combined.
Mix the wet ingredients: In a separate bowl, whisk the oil, yogurt, applesauce, egg, and vanilla until smooth.
Combine gently: Pour the wet mixture into the dry ingredients.
Stir with a spatula just until no dry streaks remain. Do not overmix.
Fold in the apples: Add the diced apple and fold gently to distribute evenly. The batter will be thick.
Portion the batter: Divide the batter among the 12 muffin cups.
Aim for about 3/4 full in each. If using, sprinkle a pinch of turbinado sugar on top of each muffin for a delicate crust.
Bake: Bake for 14–17 minutes, or until the tops spring back lightly and a toothpick comes out clean or with a few moist crumbs.
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a rack to cool completely. This helps them set and stay tender.
Serve: Enjoy warm or at room temperature.
They’re great plain, or with a thin swipe of almond butter if you’re not counting every calorie.