01 - Preheat the oven to 350°F. Lightly grease a 9x9-inch baking dish with butter or cooking spray.
02 - In a large bowl, add rolled oats, nuts (if using), baking powder, cinnamon, and salt. Mix thoroughly to distribute the baking powder and spices evenly.
03 - In a separate medium bowl, whisk together milk, maple syrup, eggs, melted butter, lemon zest, vanilla extract, and lemon juice until completely smooth and well blended.
04 - Pour the wet mixture into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined, being careful not to overmix.
05 - Gently fold the blueberries into the batter, taking care not to crush them. If using frozen blueberries, add them directly from the freezer.
06 - Pour the mixture into the prepared baking dish and spread it evenly with a spatula. Sprinkle the top with coconut sugar or brown sugar if using.
07 - Bake for 35 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean. The oatmeal should be set but still slightly moist.
08 - Let the baked oatmeal cool for 5 minutes before serving. Serve warm, optionally topped with additional milk, yogurt, or fresh berries.