01 - Preheat oven to 375°F. Lightly grease a 9x9-inch baking dish using butter or non-stick spray.
02 - In a large mixing bowl, combine oats, chopped nuts if using, baking powder, cinnamon, and salt.
03 - In a separate bowl, thoroughly whisk together milk, maple syrup or honey, eggs, melted butter or coconut oil, and vanilla extract.
04 - Add wet ingredients to the dry mixture, stirring gently until evenly incorporated.
05 - Fold mixed berries into the batter, reserving a small portion for topping.
06 - Transfer the mixture to the prepared baking dish, evenly scatter reserved berries on top, and bake for 35–40 minutes, until golden and set at the center.
07 - Allow to cool for 5 minutes before serving. Serve warm, optionally topped with extra milk or yogurt.