Preheat your oven to 350°F and grease a 8" square baking dish with a little bit of ghee or butter. Set aside.
In a large mixing bowl bowl, combine the rolled oats, walnuts, raisins, baking powder, cinnamon, salt and nutmeg. Mix until very well combined and then add the diced apple and mix again until the apple is well distributed and every piece is completely coated in spices. Set aside.
In a separate bowl, combine the milk, buttermilk, honey or maple syrup, eggs and vanilla extract. Mix vigorously with a whisk until well combined and pour right over the dry ingredients.
Stir well and transfer to the prepared baking dish. Bake for 30-35 minutes, or until the top is nicely browned and the center appears completely set.
Let cool in the pan slightly before serving; serve warm or cold, garnished with a dollop of plain Greek yogurt, a few apple slices, fresh mint leaves and chopped walnuts, if desired.
Store leftovers in the refrigerator in an airtight container for up to a week.