Preheat and prep. Heat the oven to 375°F (190°C). Line a 12-cup muffin tin with liners or lightly grease the cups.
Mix dry ingredients. In a large bowl, whisk together oat bran, flour, baking powder, baking soda, salt, cinnamon, nutmeg (if using), and brown sugar until no lumps of sugar remain.
Combine wet ingredients. In a separate bowl, whisk the egg, buttermilk, oil, and vanilla until smooth.
Grate the apples. Coarsely grate the apples (peel optional).
Lightly squeeze by hand to remove just a bit of excess juice, especially if very juicy.
Bring it together. Pour the wet mixture into the dry. Add the grated apples and any add-ins. Stir gently with a spatula until just combined.
The batter will be thick; avoid overmixing.
Fill the pan. Divide the batter into the muffin cups, filling each about 3/4 full. Sprinkle with turbinado sugar if desired.
Bake. Bake for 16–20 minutes, or until the tops spring back and a toothpick comes out with a few moist crumbs.
Cool. Let muffins rest in the pan for 5 minutes, then transfer to a rack. Enjoy warm or at room temperature.