While the pork is in the oven, slice the onions. Peel, core and wedge the apples into 1/4" -inch to 1/2" -inch wedges.
Add the oil and butter to the same pan used for the pork, once hot, add the onions and fry for a few minutes until they start to caramelize. Then add the apples and saute for another 2 minutes or until the apples also start to caramelize.
Next, add the white wine, apple cider, chicken stock and simmer for about 5 minutes. While that is simmering, chop the fresh sage and set it aside.
To thicken the sauce, mix cornstarch and water together to form a slurry. Slowly add it to the sauce. Add just enough to get the desired consistency. You may not need it all (or you may need a bit more) to get the proper consistency.
Season to taste with salt and pepper and add the butter. Reduce the heat to very low. You can also hold this sauce in a double boiler until ready to serve.
When ready to serve, add the chopped sage and gently mix to combine.