01 - Preheat oven to 375°F (190°C) and lightly grease a baking dish.
02 - In a large bowl, mix together rolled oats, baking powder, ground cinnamon, and salt.
03 - Add milk, egg, maple syrup, and vanilla extract to the dry mixture. Stir until well combined.
04 - Fold in the diced apple, ensuring even distribution throughout the mixture.
05 - Pour the oatmeal mixture into the prepared baking dish. Bake for 35 to 40 minutes, or until set and golden on top.