Preheat your oven to 375°F (190°C). Grease an 8x8-inch baking dish with cooking spray.
Mix the apple, oats, baking powder, cinnamon, and salt in a large bowl.
Whisk together the milk, eggs, vanilla extract, and honey or maple syrup in another bowl.
Add the wet ingredients to the dry ingredients and mix until well combined. Stir in the chopped nuts if using.
Pour the oatmeal mixture into the prepared baking dish and smooth out the top.
Bake in the preheated oven for 25-30 minutes or until the oatmeal is set and lightly golden on top.
Serve hot, topped with your favorite fruit, nuts, and/or sweetener. Enjoy!