Preheat oven to 375 degrees.
In a large skillet, melt the butter and add the 1/4 cup brown sugar, diced apples and cinnamon. Toss the apples around and make sure they are fully coated by butter and sugar and saute for about 3 minutes. Do not stir during the first 3 minutes, so that the apples caramelize on one side. After 3 minutes toss the apples again and let them caramelize for another 3 minutes on the other side. Let cool.
Butterfly the pork loin, basically cut the pork down the center, without completely cutting through, so when you open the two halves they resemble a butterfly. Use your meat mallet to flatten it a bit.
Take the cooled apples and spread them down the center of the pork loin and bring the 2 sides up around filling the meat. Use butcher twine to tie around the roll tightly at 1 inch intervals.
Season the stuffed pork with salt and pepper. In a large skillet heat the oil. Sear the pork loin on all sides.
Transfer the pork loin to a large oven proof pot. Mix the orange juice with the brown sugar and pour over the pork loin.
Roast uncovered for about 1 hour and 30 minutes or until done to your preference. Usually you would roast for about 20 minutes per pound, so if your pork is smaller or bigger, adjust as necessary.
Remove from the oven and wrap it with foil and let it sit for 15 minutes before slicing so all the juices don't fall out of the pork.
Slice it and serve with mashed potatoes.