Preheat oven to 350°F. Lightly coat a 7-by-11-inch baking dish with cooking spray. Chop peeled, cored apples into bite-size pieces to equal 2 cups. Slice the remaining apple into ¼-inch-thick half-moons.
Whisk 1½ cups milk, ½ cup applesauce and ¼ cup maple syrup together in a large bowl until well blended. Fold in the chopped apples, 2½ cups oats, 2 tablespoons flaxseed, 1 tablespoon oil, 1½ teaspoons cinnamon, 1 teaspoon vanilla, 1 teaspoon baking soda and ⅛ teaspoon nutmeg until well combined.
Transfer the mixture to the prepared baking dish; arrange the sliced apples decoratively over the top. Bake until golden on top and the apples are tender, 40 to 45 minutes.
