In a third bowl, whisk together the milk, maple syrup, egg, pure vanilla extract, and room temperature melted butter until thoroughly mixed together. Set aside.
Add a good layer of the prepared fruit to generously cover the bottom of the buttered baking dish. Add the oatmeal and spread it to cover the layer of fruit. Pour the milk mixture all over the oatmeal, and tilt the pan to encourage the milk to flow into all corners and throughout the oatmeal. Bang the pan against the counter to make sure the liquid has flowed completely through the rolled oats and fruit. Add the remaining fruit and chopped nuts to cover the top of the oatmeal.