In a small bowl, combine the corn starch and apple cider to create a slurry, set aside.
6 tbsp corn starch, 3 tbsp apple cider
Melt the butter in a large, deep skillet (about 7qt).
8 tbsp butter
Add the apples, sugars, cinnamon, nutmeg, allspice, cloves and salt. Gently turn and redistribute the apples until they are coated in the butter, sugar and spices. (If there is not enough room in the skillet to move the apples around at this stage, you can toss the apples, sugars, cinnamon, nutmeg, allspice, cloves and salt together in a large bowl before adding to the pan)
15 large apples, 1 1/2 cup toasted/caramel sugar, 1 1/2 cup brown sugar, 1 1/2 tbsp cinnamon, 2 tsp nutmeg, 1/2 tsp allspice, 1/2 tsp cloves, 1 1/2 tsp salt
Cook the apples, stirring and redistributing occasionally until they are tender - they should be easy to pierce with a fork but still retain a bit of a snap to the bite.
Remove the apples from the juices making sure to strain as much juice from the apples as possible to retain in the pan.
Whisk the corn starch slurry and the apple cider caramel into the juices in the pan. Cook over medium heat until very thick and syrupy.
1 cup caramel
Return the apples to the spiced caramel syrup. Stir well to coat all the apples evenly with the syrup. Remove from the heat and cool completely.
If you are not using the filling immediately, store it in an air tight container in the fridge until you are ready to use it. Allow it to return to room temperature before using in a pie or tart - if it is cold it will impact how the crust cooks.
Notes
For pie filling, I recommend thinly slicing the apples. For a topping or to swirl into batter, I recommend dicing the apples - the size of the dice will depend on exactly what you are using the apple pie filling for. Most of the time I prefer a small dice (about 1/4 inch) for a topping, but for a swirl in batter, I prefer a fine dice, or brunoise cut, (about 1/8 inch).This makes a lot of apple pie filling (about 5qt), this is the amount I would use for a very full 15 x 10 inch slab pie, or nearly the amount that I would use for a 9 1/2 - 10 inch deep dish pie (mounded above the rim). For reference, a standard 9 inch pie, filled level with the top of the pie plate, will take about 1/3 of this recipe. If you are making a smaller dessert you can either cut the recipe down, or you can use this recipe as is to make ahead and can the filling in 1qt jars for later use. If you do plan to can the filling, you do not need to cool the filling before canning it.I like to use this same recipe as the base for a cranberry apple pie filling: place 2 cups of sweetened dried cranberries in a heat safe bowl. Add just enough either apple or cranberry based juice to cover the dried berries. Heat for 1-2 minutes in the microwave or until the mixture is just barely at a simmer. Set aside while making the apple filling. When you stir the cooked apples back into the syrup, add the rehydrated cranberries as well. If there is any excess liquid that was not absorbed by the cranberries, discard it before adding to the apple mixture. Use, or can, the cranberry apple pie filling exactly the same way you would use the original apple pie filling.