Prep the pan and oven. Heat oven to 350°F (175°C). Line a 12-cup muffin tin with liners or grease well.
Mix dry ingredients. In a large bowl, whisk flour, protein powder, baking powder, baking soda, cinnamon, nutmeg, and salt until combined and lump-free.
Whisk wet ingredients. In a separate bowl or large measuring cup, whisk eggs, applesauce, milk, maple syrup, oil, and vanilla until smooth.
Combine gently. Pour wet into dry. Stir with a spatula until just combined.
Do not overmix—some small flour streaks are okay.
Fold in apples. Add the chopped apples and fold gently to distribute evenly.
Portion the batter. Divide among the muffin cups, filling each about 3/4 full. Sprinkle with oats and turbinado sugar if using.
Bake. Bake 16–20 minutes, or until the tops spring back and a toothpick comes out mostly clean 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 set the structure.