Preheat the oven to 375°F (190°C) and lightly grease an 8-inch square baking dish.

In a large mixing bowl, combine the rolled oats, half of the chopped walnuts, baking powder, cinnamon, and salt.

In a separate medium bowl, whisk together the almond milk, egg, maple syrup, melted coconut oil, and vanilla extract until well combined.

Arrange half of the fresh blueberries in a single layer at the bottom of the prepared baking dish.

Spread the dry oat mixture evenly over the blueberries.

Slowly pour the liquid mixture over the oats, ensuring all oats are submerged; tap the dish gently on the counter to remove air bubbles.

Scatter the remaining blueberries and walnuts over the top of the mixture.

Bake for 35 to 40 minutes until the top is golden brown and the oatmeal has set.

Remove from the oven and let cool for 5 minutes before slicing and serving.