01 - Preheat the oven to 350°F. Lightly grease a 9x9-inch baking dish with butter or cooking spray.
02 - In a large bowl, combine rolled oats, nuts (if using), baking powder, cinnamon, and salt. Mix thoroughly to ensure even distribution of leavening and spices.
03 - In a separate bowl, whisk together milk, maple syrup, eggs, melted butter, lemon zest, vanilla extract, and lemon juice until fully blended and smooth.
04 - Pour the wet ingredients into the dry ingredients. Stir gently until just combined, being careful not to overmix as this can affect the final texture.
05 - Gently fold in the blueberries using a spatula, taking care to distribute them evenly without crushing the fruit.
06 - Transfer the mixture to the prepared baking dish and spread evenly across the surface. Sprinkle with coconut sugar or brown sugar if a crunchy topping is desired.
07 - Bake for 35 minutes, or until the top is golden brown and the oatmeal is set in the center. A toothpick inserted should come out mostly clean.
08 - Allow the baked oatmeal to cool for 5 minutes before serving. Serve warm, with extra milk or yogurt on the side if desired.