Preheat your oven to 375°F (190°C) and lightly grease a baking dish.
In a mixing bowl, combine the oats, cinnamon, baking powder, and salt. Stir until evenly mixed.
In another bowl, whisk together the beaten egg, milk, and maple syrup until smooth.
Pour the wet mixture into the dry oats and stir gently to combine, making sure everything is evenly coated.
Fold in the chopped apple pieces, distributing them evenly throughout the mixture.
Pour the mixture into the prepared baking dish, spreading it out evenly with a spatula.
Sprinkle a little extra cinnamon on top for a fragrant crust, and optionally add a pinch of sugar.
Bake in the preheated oven for about 35 minutes, until the top is golden and the mixture is set around the edges.
Remove from the oven and let it cool slightly before serving, allowing the flavors to relax and the texture to firm up.
Slice into squares and enjoy this comforting, hearty breakfast with a sprinkle of extra cinnamon or a drizzle of syrup if you like.