Cook the apple in a medium sized skillet set over medium heat, until slightly softened. Add the apple juice and continue cooking until all the juice is evaporated. Mash the apple roughly with a with a spoon, if necessary, so the mixture looks like real chunky applesauce. Kill the heat.
Add the rest of the ingredients to the pan, except for the pork tenderloin, and mix until very well combined. Let cool for a few minutes while you work on butterflying the pork tenderloin.
To do that, slice the tenderloin lengthwise, while holding your blade parallel to the cutting board, about 3/4 of the way through. Open the tenderloin like a book and press it down with your fingers to flatten it up.
Mound the apple mixture in the center of the meat and then spread it as evenly as you can across the entire surface of the tenderloin.
Fold the tenderloin onto itself and tie it up nice and tight with butcher's twine.
Wrap the stuffed roast in plastic film and refrigerate it for at least 2 hours, or up to the next day.
When you are ready to cook your meat, preheat your outdoor grill to medium-high (400°F), then cook the roast for 15 to 20 minutes, turning it from time to time, until a meat thermometer inserted in the thickest part of the roast registers 145°F. Avoid placing the exposed stuffing, if present, directly on the grill.
Let the stuffed tenderloin rest, lightly tented with aluminum foil, for about 5 minutes, then remove the twine, carve and serve.
Notes
You could also cook this in a 400°F oven. Just sear the tenderloin in a very hot pan first, then cover lightly with aluminum foil and transfer to the oven; bake for about 20 minutes, or until the temperature on a meat thermometer reads 145°F.