01 - Preheat the oven to 350°F and lightly grease a 9x9-inch baking dish with cooking spray or butter.
02 - In a large bowl, combine rolled oats, nuts, baking powder, cinnamon, and salt. Mix thoroughly to distribute evenly.
03 - In a separate bowl, whisk together milk, maple syrup, eggs, melted butter, lemon zest, vanilla extract, and lemon juice until fully incorporated.
04 - Pour the wet ingredients into the dry ingredients and stir until just combined. Avoid overmixing to prevent tough texture.
05 - Gently fold in the blueberries, 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 with coconut sugar or brown sugar if desired.
07 - Bake for 35 minutes, or until the top is golden brown and the oatmeal is set. A toothpick inserted in the center should come out moist but not liquidy.
08 - Allow to cool for 5 minutes before serving. Serve warm, with extra milk or yogurt if desired.