Preheat oven to 375°F. Line a 12-cup muffin pan with parchment or cupcake liners, or lightly grease with cooking spray. Set aside.
Puree oats in a blender or food processor until they reach a fine flour-like consistency. Add in the pumpkin pie spice, baking soda and sea salt, and pulse until the mixture is evenly combined. With the power turned off, add in the diced apple. Then use a spoon to toss until the apple is completely coated in the flour mixture. Set aside.
In a separate large mixing bowl, whisk together the eggs, milk, applesauce, maple syrup, coconut oil and vanilla extract until evenly combined. Fold the dry ingredients in with the wet ingredient mixture, and stir until the mixture is just combined. (Try to avoid over-mixing.)
Portion the batter evenly into the prepared baking cups. Then sprinkle a generous pinch of turbinado sugar on top of each muffin, if you would like.
Bake for 15-18 minutes, or until a toothpick inserted in the center of the muffins comes out clean. Remove from the oven and place the pan on a cooling rack for 5 minutes.
Serve warm. Or let the muffins cool to room temperature, then store in a sealed container for up to 3 days, or freeze for up to 3 months.