Preheat your oven to 375°F and lightly grease an 8x8-inch baking dish.
In a large bowl, whisk together the milk, egg, maple syrup, melted butter, and vanilla until well combined.
Add the oats, baking powder, cinnamon, and salt. Stir until everything is evenly mixed.
Gently fold in the berries, reserving a small handful to sprinkle on top.
Pour the mixture into the prepared baking dish and spread it evenly.
Scatter the remaining berries over the top.
Bake for 35 minutes, or until the center is set and the top is lightly golden.
Let cool for 5 to 10 minutes before slicing and serving.