Preheat the oven to 350 degrees and prepare a nine-by-13-inch baking pan by spraying it with cooking spray. Set it aside.

In a large mixing bowl, beat the butter until it is smooth. Beat in the sugar and continue mixing on high until the mixture is light and fluffy. Beat in the eggs and vanilla.

Beat in the flour, cinnamon, baking soda, salt and nutmeg until everything is just combined.

Fold in the diced apples and walnuts by hand.

Spread the batter into your prepared pan and bake for 50 to 55 minutes or until a toothpick inserted into the center comes out clean.

While the bars cook, prepare the topping.

Begin by lightly toasting the walnuts for several minutes in a dry skillet over medium heat. Just let them get slightly browned and warmed through to really release some of the yummy oil in the nuts.

Then combine all of the topping ingredients in a bowl, mixing well.

When the bars come out of the oven, spoon the topping mixture evenly over the top, and bake for another five minutes.

Let the bars cool completely before cutting and serving. Store any leftovers in an airtight container.