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. Whisk briefly to evenly distribute ingredients.
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 ingredients into the dry ingredients. Stir with a spatula or wooden spoon until just combined, being careful not to overmix.
05 - Gently fold in the blueberries using a spatula, taking care not to crush them. If using frozen blueberries, add them directly from the freezer.
06 - Transfer the mixture to the prepared baking dish and spread evenly with a spatula. Sprinkle the top evenly with coconut sugar or brown sugar if using.
07 - Bake for 35 minutes, or until the top is golden brown and the center is set. A toothpick inserted into the center should come out mostly clean.
08 - Allow to cool for 5 minutes before serving. Serve warm, optionally topped with additional milk or yogurt.