Peel, core and slice apples.   Place the sliced apples in a bowl and stir in the juice of a lemon to prevent the apples from browning.  Set the bowl aside.

In a 4 quart dutch oven [or other sturdy pot], melt the butter over medium heat and let it continue to cook until the butter begins to bubble. Continue  cooking and stirring constantly until the butter develops brown flakes and has a nutty smell.  About 5-7 minutes. 

Whisk in the brown sugar and let the butter/sugar mixture cook for 3-5 minutes stirring occasionally. The mixture should be nearing the caramel stage.  If you notice that the mixture has beginning 

Toss in the apples, cinnamon, nutmeg and vanilla extract.  The apples will release their juice, but if there is not enough liquid in the pan, add in a splash of water.

Stir constantly and let the mixture come to a boil.  Cook the apples until they are slightly softened.  Timing will vary depending on how thin the apples are sliced. About 5-7 minutes.  

While the apples are cooking, make a slurry with 1 tablespoon of cornstarch and  one tablespoon of water in a small bowl.  Pour the slurry into the apple mixture to thicken the filling.  Let the pie filling cook for an additional minute to activate the cornstarch slurry.  

Remove the apple filling from the heat and cool completely before adding to the pie crust.