In a 10-inch oven proof skillet, melt 1 tablespoon butter over medium heat. Add the apples, coconut sugar, and ¼ teaspoon cinnamon and stir with a wooden spoon until apples begin to soften, about 5 minutes. Set aside.
In a mixing bowl, combine the oats, rye, barley, raisins, baking powder, remaining 1 teaspoon cinnamon, and salt. Pour evenly over the apples.
In the same bowl, whisk together the milk, egg, vanilla, and maple syrup. Whisk in the melted butter. Carefully and evenly pour over the oat mixture. Lightly tap pan on the counter to remove any bubbles. Arrange apples slices and scatter walnuts over the mixture.
Bake until top begins to turn golden and oatmeal is set, about 30-40 minutes.