Preheat your oven to 325°F (163°C) and lightly grease an 8x12-inch, 9x13-inch, or 9x9-inch baking pan with nonstick cooking spray. If you plan to make it ahead, you can skip preheating the oven for now and refrigerate the dish overnight.
In a large bowl, whisk together the oats, baking powder, cinnamon, nutmeg, salt, and brown sugar.
In a separate bowl or large liquid measuring cup, whisk together the milk, eggs, and vanilla extract.
Pour the wet ingredients over the dry ingredients and stir in the melted butter or coconut oil until fully combined.
Arrange the diced apples evenly on the bottom of the prepared baking pan. Pour the oatmeal mixture over the apples.
If making ahead, cover and refrigerate overnight. If baking right away, uncover the dish before placing it in the oven.
Bake for 35-45 minutes, or until the top is golden and the oatmeal is set and no longer jiggly in the center.
Serve warm or at room temperature. You can add a splash of milk on top for extra creaminess.
