Preheat oven to 350 degrees F. Coat a 13- x 9-inch baking dish with nonstick cooking spray.
Empty the apple pie filling into a large mixing bowl. Use a metal spoon to break up the apple slices into smaller pieces. Add the eggs, water and oil, and stir to combine. Add the cake mix, 1 teaspoon cinnamon, and the nutmeg and stir until well combined. Spread the batter in the prepared baking dish.
In a small mixing bowl, combine the walnuts, sugar, flour, and remaining ½ teaspoon cinnamon. Add the melted butter and toss to combine. Sprinkle the mixture evenly over batter.
Bake 40 to 45 minutes, or until a toothpick inserted in the center of the cake comes out with just a few moist crumbs and the streusel is golden brown. If the streusel is getting too dark, lightly place a sheet of foil over the top of the baking dish towards the end of the baking time.
Cool in the baking dish for at least 15 minutes before serving. Delicious served slightly warm with a scoop of vanilla ice cream!
This is a very moist cake and will be at its best if consumed within a day or two. Cover and refrigerate any leftovers beyond that point to keep it nice and fresh.