Preheat oven to 375°F (190°C) and grease an 8×8 or 9×9-inch baking dish.
In a large bowl, combine oats, chia seeds, baking powder, salt, cinnamon, and nutmeg.
Add maple syrup, mashed banana, eggs, milk, melted butter, and vanilla extract. Mix well.
Fold in blueberries and almonds gently.
Pour mixture into prepared baking dish and spread evenly.
Top with extra blueberries and almonds if desired.
Bake for 30–35 minutes until the center is set.
Let cool for 10 minutes before serving this healthy blueberry baked oatmeal.