Preheat the oven to 350°F then lightly grease a muffin tin for 10 muffins.
In a small bowl, whisk 1 tablespoon ground flax with 3 tablespoons water. Set aside to thicken.
In a large bowl, combine the milk, apple cider vinegar, applesauce, maple syrup, vanilla, and almond butter (if using coconut oil, add to bowl when you add dry ingredients). Whisk until smooth.
In a separate large bowl, whisk together oat flour, almond flour, cinnamon, baking powder, baking soda, and salt. Add the diced apple and stir to coat.
Next add the dry ingredients and the flax egg to the bowl with the wet, folding together as you combine. If you're using coconut oil instead of almond butter, add it now. You want to make sure there isn't any flour left that needs to be incorporated but also fold just until everything is combined, being careful not to overstir.
Scoop the batter into the greased muffin so that each is about 3/4 of the way full. Arrange extra diced apple on top of each one then sprinkle with granola. Bake in the oven for 20-25 minutes, until golden on the edges and firm on top. Allow to cool completely before serving. Store leftovers in an airtight container in the refridgerator for up to 1 week.