01 - Preheat oven to 350°F. Lightly grease a 9x9-inch baking dish with cooking spray or butter.
02 - In a large mixing bowl, combine rolled oats, nuts (if using), baking powder, cinnamon, and salt. Mix thoroughly to distribute spices evenly.
03 - In a separate bowl, whisk together milk, maple syrup, eggs, melted butter, lemon zest, vanilla extract, and lemon juice until smooth and fully blended.
04 - Pour wet ingredients into dry ingredients. Stir until just combined, being careful not to overmix.
05 - Gently fold blueberries into the batter, taking care not to crush them if using fresh berries.
06 - Pour mixture into prepared baking dish and spread evenly. Sprinkle with coconut sugar or brown sugar if desired for a caramelized topping.
07 - Bake for 35 minutes or until top is golden brown and center is set. A toothpick inserted in the center should come out clean.
08 - Let cool for 5 minutes before serving. Serve warm, optionally topped with additional milk or yogurt.