The pork loin package with come with two tenderloins.
Slice pork starting about a third of its thickness and slice about halfway through. You're going to turn the pork so that you're creating a long, flat piece of pork while cutting. (I'm sure that makes no sense, but it will once you're doing it)
Combine apple, onion, water, and stuffing mix into a mixing bowl and let the stuffing absorb some of the water.
Spread half of stuffing mix over one spread tenderloin and the other half on the other.
Using kitchen twine, roll up the pork and tie it tightly. 3 or 4 ties should work.
Place pork tenderloin, seam side down in greased dish.
Spread butter over top of pork and sprinkle with a little salt and pepper.
Bake at 350 for about 1 hr 15 min.
Recipe by The Shirley Journey at https://theshirleyjourney.com/apple-stuffed-pork-tenderloin/