Lightly butter a 9" square baking dish., In a large bowl, combine the oats, flax, sugar, spices, and salt. Set aside., In a separate bowl or large measuring cup, mix together the milk, boiled cider, applesauce, butter, and vanilla., Stir the wet ingredients into the dry until evenly moistened. Stir in the apple cubes and nuts., Spoon the oatmeal mixture into the prepared pan, spreading it into an even layer., Cover the pan with plastic and refrigerate the oatmeal overnight., Next morning, preheat the oven to 350°. Top the oatmeal decoratively with the thinly sliced apple and sprinkle with Demerara sugar for extra sweetness and crunch, if desired., Bake the oatmeal for 55 to 65 minutes, until the edges are bubbly; the center will still appear slightly wet. The apples on top should be tender., Remove the oatmeal from the oven, and allow it to cool for 5 to 10 minutes before serving., Store baked oatmeal in the refrigerator and reheat individual servings with a little water or milk in a microwave or over a burner.