Preheat the oven to 375°F (190°C). Lightly grease an 8x8-inch baking dish with oil or line it with parchment paper to prevent sticking and make cleanup easier.

In a medium mixing bowl, add the rolled oats, baking powder, cinnamon, salt, and chia or flax seeds if using. Stir everything together with a whisk or spoon so the spices and leavening agent are evenly distributed throughout the oats.

In a separate bowl, mash the ripe banana until mostly smooth. Add the almond milk, maple syrup, vanilla extract, and melted coconut oil. Whisk until the mixture is well combined and slightly creamy.

Pour the wet ingredients into the bowl with the dry ingredients. Stir gently until the oats are fully coated and the mixture looks evenly hydrated. Avoid overmixing; just ensure everything is incorporated.

Fold in the blueberries and chopped nuts. If using frozen blueberries, you can add them directly without thawing. This step adds texture, bursts of flavor, and extra nutrients to the oatmeal.

Pour the mixture into the prepared baking dish. Use a spatula or spoon to spread it evenly so it bakes uniformly. You can sprinkle a few extra berries or nuts on top for presentation.

Cut into squares and serve warm. It pairs wonderfully with extra vegan milk, a drizzle of maple syrup, or a spoonful of dairy-free yogurt.