Peel and slice the apples into thin pieces.
In a bowl, combine apples with lemon juice, sugars, cinnamon, nutmeg, and salt.
Let the mixture sit for 10 minutes to release juices.
Mix cornstarch with a few tablespoons of the apple mixture to create a slurry.
Heat a skillet over medium heat and add the apple mixture.
Cook for 5-7 minutes, stirring occasionally until apples soften.
Add the cornstarch slurry and stir until thickened.
Remove from heat and stir in vanilla extract and butter.
Let the filling cool before using in a pie crust.
Your apple pie filling is ready for use!