Preheat Oven:
Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish or line it with parchment paper.
Mix Dry Ingredients:
In a large mixing bowl, combine the oats, baking powder, cinnamon, nutmeg, and salt. Stir to distribute evenly.
Prepare Wet Ingredients:
In another bowl, whisk together the milk, eggs, maple syrup (or honey), and vanilla extract until smooth.
Combine Mixtures:
Pour the wet ingredients into the bowl with the dry ingredients. Stir well to combine.
Add Apples and Extras:
Fold in the diced apples, along with any optional add-ins like nuts or dried fruit.
Bake:
Transfer the mixture to the prepared baking dish. Spread it out evenly with a spatula. Bake for 35–40 minutes or until the top is golden brown and the oatmeal is set.
Cool and Serve:
Let the oatmeal bake cool for 5–10 minutes before slicing. Serve warm or at room temperature, plain or topped with yogurt, additional maple syrup, or fresh fruit.