01 - Preheat the oven to 350°F. Lightly grease a 9x9-inch baking dish with cooking spray or butter.
02 - In a large bowl, combine rolled oats, nuts (if using), baking powder, cinnamon, and salt. Mix thoroughly to distribute the spices and leavening agent evenly.
03 - In a separate bowl, whisk together milk, maple syrup, eggs, melted butter, lemon zest, vanilla extract, and lemon juice until the mixture is smooth and fully blended.
04 - Pour the wet ingredients into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined, being careful not to overmix.
05 - Gently fold in the blueberries, distributing them evenly throughout the batter. If using frozen berries, do not thaw first to prevent excess moisture.
06 - Transfer the mixture to the prepared baking dish and spread it into an even layer. Sprinkle the top with coconut sugar or brown sugar if using.
07 - Bake for 35 minutes, or until the top is golden brown and the oatmeal is set in the center. A knife inserted near the center should come out clean.
08 - Allow the baked oatmeal to cool for 5 minutes before serving. Serve warm, with additional milk or yogurt if desired. Leftovers can be refrigerated and reheated or served cold.