Preheat the oven to 350°F.
Lightly coat a 9×13-inch baking dish with oil spray.
In a large bowl, combine the oats, cinnamon, nutmeg, baking powder, and salt.
In a separate bowl, whisk together the milk, eggs, coconut sugar (or substitutes), and vanilla.
Pour the wet ingredients into the dry ingredients and stir gently until just combined.
Spread the mixture evenly in the prepared baking dish and sprinkle the diced apples evenly over the top, then lightly fold them in.
(Optional) Drizzle honey or maple syrup over the top for extra sweetness and caramelization.
Bake for 40–45 minutes, until the top is set and golden brown.
Let cool for a few minutes before serving.