01 - Preheat oven to 375°F. Lightly grease a 9x9-inch baking dish.
02 - In a large bowl, combine rolled oats, chopped nuts if using, ground cinnamon, baking powder, and salt.
03 - In a separate bowl, whisk together milk, eggs, maple syrup or honey, melted butter or coconut oil, and vanilla extract until fully blended.
04 - Scatter half the mixed berries evenly across the bottom of the prepared baking dish.
05 - Pour the oat mixture over the berries and spread evenly.
06 - Pour the wet mixture over the oats, ensuring all oats are moistened. Gently tap the dish to settle.
07 - Sprinkle the remaining berries evenly on top.
08 - Bake in the preheated oven for 35 to 40 minutes, until the center is just set and the surface is golden brown.
09 - Allow to cool for 5 minutes before serving. Serve warm, optionally garnished with Greek yogurt and extra fresh berries.