Heat a large pan on the stove to medium heat. Pour oil into pan, add celery root with a pinch of salt and cook for about 5 – 6 minutes. Add diced apples into pan and cook another 2 – 3 minutes, stirring occasionally. Add ¼ cup Maple Apple Jam and thyme, remove from heat and allow to cool.
Preheat oven to 400°F.
Using a long, thin blade such as a slicer, poke a hole through the pork loin going from one cut end to the other.
Using your thumbs, stuff a small amount of the apple/celery root mixture into each end of the hole until the entire loin is stuffed. You can truss the roast with butcher twine after stuffing if you are comfortable and capable, although it is not necessary.
Place in a roasting pan with a rack, season with salt and pepper, and pour the remaining Maple Apple Jam over the top of the pork. Roast in oven at 400°F for 15 minutes, then turn the oven down to 300°F and continue to cook until thermometer reads 150°F, or until you have reached desired doneness. Allow to rest for 10 – 15 minutes prior to slicing.