Pre-heat the oven to 350° F – spray a 8 x 12 inch oven dish with cooking oil.
Heat the Oil in a medium pan – saute the Onion, Apples and Sage until tender – stir often – remove from heat – stir the Walnuts into the warm Apple mixture and let cool.
Spoon the Apple stuffing into the slits of the Pork Tenderloin – press a little, but make sure not to over stuff the Pork – close the opening and wrap the Bacon around the Tenderloin – secure with toothpicks.
Bake 40 – 45 minutes or until done – remove from oven and let sit for 10 minutes before slicing.
Serve with a Gravy of your choice, mashed potatoes and vegetable sides.