Preheat the oven to 400ºF. Line a muffin tin with 12 paper liners or grease with non-stick cooking spray. Set aside.
In a large mixing bowl, combine quick-cooking oats, all-purpose flour, light brown sugar, ground cinnamon, nutmeg, baking powder, baking soda, and salt.
In another bowl, whisk together unsweetened applesauce, whole milk, vegetable oil, egg, and vanilla extract.
Pour combined wet ingredients over the dry ingredients and stir gently just until combined. Don’t overmix the batter or gluten will develop and the muffins will turn out dense.
Add peeled, cored, and chopped apples. Gently fold in.
Divide the batter evenly into the prepared 12 paper liners or greased muffin tin.
Prepare the oatmeal crumb topping. In a small bowl, combine quick-cooking oats, light brown sugar, melted butter, and cinnamon. Sprinkle on top of each muffin.
Bake for 15 minutes or until golden brown and a toothpick inserted in the center, if it comes out clean.
Remove from the oven and let them cool slightly in a muffin tin. Then, transfer to a cooling rack. Enjoy!