01 - Preheat oven to 350°F. Lightly grease a 9x9-inch baking dish with butter or cooking spray.
02 - In a large mixing bowl, whisk together rolled oats, chopped nuts if using, baking powder, cinnamon, and salt until evenly distributed.
03 - In a separate bowl, whisk milk, maple syrup, eggs, melted butter, lemon zest, vanilla extract, and lemon juice until smooth and well blended.
04 - Pour the wet mixture into the dry ingredients and stir gently until just combined, being careful not to overmix.
05 - Gently fold in the blueberries, distributing them evenly throughout the batter while keeping them intact.
06 - Spread the mixture evenly in the prepared baking dish. Sprinkle coconut sugar or brown sugar over the top if desired for added sweetness and crunch.
07 - Bake for 35 minutes, or until the top is golden brown and the oatmeal is set. A knife inserted in the center should come out clean.
08 - Allow to cool for 5 minutes before serving. Serve warm with additional milk or yogurt on the side if desired.