Preheat oven to 375 degrees F. Grease an 8-inch or 9-inch baking dish; set aside.
In a medium bowl, stir together the oats, half of the walnuts, cinnamon, baking powder and salt; set aside.
In a separate bowl, whisk together the milk, maple syrup, egg, melted butter and vanilla.
Arrange the sliced apples in a single layer on the bottom of the baking dish. Spread the oat mixture over the apples, in an even layer. Slowly drizzle the milk mixture all over the top of the oats. Tap the baking dish a couple of times to disperse the milk among the oats. Sprinkle the top with the remaining walnuts.
Bake for 35-45 minutes, or until the top is golden brown and the oatmeal mixture is set. Remove from the oven and let sit for at least 5 minutes.
Recipe by Sprung At Last at https://sprungatlast.com/2013/03/apple-cinnamon-baked-oatmeal/