For the crumb topping: in a small bowl combine the oats, flour, brown sugar, and cinnamon. Add the butter to the bowl. Cut with a pastry cutter (or 2 knives, or the back of a fork) until well combined and mixture resembles coarse meal. Place in the refrigerator while you prepare the muffins
Preheat the oven to 350°F. Line a muffin tin with muffin liners or silicone liners (or grease the muffin tin with butter or oil).
In a small bowl add the milk and apple cider vinegar, stir well and let sit for at least two minutes before adding to the wet ingredients.
In a medium sized bowl, mix together all of the dry ingredients (whole wheat flour to nutmeg)
In a large bowl, add the milk-apple cider mixture, apple sauce, eggs, vanilla extract, and brown sugar and whisk until well combined. Add the melted coconut oil and stir well until well incorporated.
Add the dry ingredients to the wet ingredients. Using a large spoon mix until just combined (a few lumps are ok). Gently fold in the diced apples. *
Divide the batter evenly among 12 muffin cups, filling almost to the top of each muffin tin. Then sprinkle the tops of each muffin with the crumb mixture, dividing evenly. Lightly press the crumbs down into the batter.
Bake the muffins 23-25 minutes until the tops are set, or until a toothpick inserted into muffins comes out clean.
Remove the muffins from the oven and let cool for five minutes in the muffin pan. After five minutes, place the muffins onto wire rack to cool completely before storing in an airtight container.