Pre-heat oven to 180°F.
Spray a 9 x 13-inch baking dish with cooking oil.
In a medium pan, heat the oil and sauté the onion, apples and sage until tender; stir often until cooked.
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.
Place in pre-heated oven and bake 40 – 45 minutes or until done.
Remove from oven and let sit for 10 minutes before slicing.
