Preheat oven to 350 degrees. Line a 9x13 baking dish with parchment paper and lightly spay with non-stick cooking spray; set aside.
In a large bowl add the flour, old fashioned oats, brown sugar, baking soda and salt. Whisk together to combine. Add 1 cup plus 2 tablespoons cold cubed butter and mix with a fork or a pastry blender, until mixture resembles coarse crumbs. Scoop out 2 cups and place in a medium bowl; set aside for the topping later.
Press remaining flour mixture into the prepared baking dish. Spread diced apples evenly over the top of the flour mixture.
In a small saucepan, add brown sugar, cornstarch, water, vanilla extract, cinnamon, nutmeg and remaining 2 tablespoons butter. Mix to combine and bring to a boil. Cook and stir for 2 minutes or until mixture thickens and is bubbly. Spread over apples.
Crumb together reserved flour mixture and sprinkle over apples. Making sure to make large crumbs.
Bake for 35-45 minutes, or until top is lightly browned and set. Let bars cool until cutting into them.
Serve & Enjoy!