Peel skins and remove core of each apple. Once prepared thinly slice apples with a mandolin for accuracy. Or, if done by hand, try to ensure they are the same thickness for even cooking. I aim for slices that are about 1/4-inch thick.
Toss apples with sugar and let sit for at least 15 minutes (the longer the better, even up to 2 hours is great), so juices can be drawn out.
Pre-heat oven to 375°F and move rack to lowest setting so it is closest to the heating element. Place cookie sheet on this rack so it can heat while oven does.
Line a 9-inch pie plate with refrigerated pie pastry dough or homemade crust. Crimp or flute edges. Alternatively, you can use a pre-made pie crust that is in a tin pie pan. Set aside.
Use a slotted spoon to remove apples from excess liquid and pat dry.
Discard juices from bowl. Then, combine apples, butter, sour cream, lemon juice, sugar, flour, cinnamon and nutmeg in a bowl. Spoon filling into pie crust.
To make the crumb topping, use a separate bowl to combine flour and sugar. Once mixed, cut in the butter using a fork until the mixture resembles coarse crumbs or clumpy sand.
When you can no longer see the butter and the mixture has a crumbly texture sprinkle the crumb topping over the entire pie filling.
Place pie in the oven and on the heated cookie sheet. Bake pie at 375°F for 45-50 minutes or until the filling is bubbly and apples are tender. Once you notice bubbling, let the pie cook for an additional 5-6 minutes.
After removing pie from the oven, allow it to cool on a wire rack for a full 3 hours, so it can come to room temperature and juices can set before serving. Serve pie with a scoop of vanilla ice cream or a dollop of whipped cream. Enjoy!