Preheat your oven to 350 degrees F. Spray a 9 x 13 baking dish with cooking spray and set aside.
In a large mixing bowl, toss the apples with the granulated sugar, brown sugar, cinnamon, and nutmeg until evenly coated. Pour into the prepared baking dish and spread into an even layer.
In a medium mixing bowl, stir together the flour, sugar, salt, and rolled oats. Use a fork or pastry cutter to cut the cold butter into the dry ingredients until it looks like small peas.
Sprinkle the topping over the apples in an even layer.
Bake for 1 hour until the top is browned and crispy and the apple filling is bubbling.
Remove from the oven and serve warm as is or with a scoop of vanilla ice cream.