First, preheat your oven to 375°F (190°C). Lightly grease an 8-inch square baking dish with butter or cooking oil. This prevents sticking and makes serving easier.
In a large mixing bowl, combine the rolled oats, baking powder, cinnamon, and salt. Stir thoroughly to distribute the seasonings evenly. This step helps every bite develop a consistent flavor.
In a separate bowl, whisk together the milk, eggs, maple syrup, vanilla extract, and melted butter. If you're using mashed banana, add it to this mixture. Whisk until everything is well combined.
Pour the wet ingredients into the bowl of dry ingredients. Stir gently until the oats are evenly coated. Fold in the blueberries and chopped nuts, if desired.
Pour the oatmeal mixture into the prepared baking dish. Spread it evenly using a spatula. Add a few extra blueberries or a light sprinkle of cinnamon on top for an attractive finish.
Place the dish in the preheated oven and bake for 35–40 minutes. The top should look lightly golden, and the center should be set. If the middle still appears excessively wet, bake for another 5 minutes and check again.
Remove the dish from the oven and let it rest for 5–10 minutes. This allows the oatmeal to firm up for easier slicing. Finally, serve warm with yogurt, fresh fruit, milk, or maple syrup.