Pre-heat the oven to 425°F.
In a large bowl combine apples, blackberries, onion, fresh rosemary, breadcrumbs, melted butter. Season with salt and pepper, mix well.
Butterfly the pork loin by making a slit down its length, cutting just deep enough so that the loin opens up to lie flat like a book. Do not cut all the way through. Spoon in the stuffing mixture and spread evenly onto the meat. Close up the loin and, using strings, tie at even intervals so it assumes its original shape. Push in any stuffing that escapes from the ends.
Rub the joint all over with sea salt and freshly ground black pepper and place the three sprigs of rosemary on top under the twine. Place in a roasting tray, any leftover filling can be placed under the pork on the roasting tray. Roast for 25 minutes, then reduce the heat in the oven to 350°F and cook for 20 minutes per 16 ounces.
For the apple and blackberry sauce – place the apples, blackberries, sugar and butter in a pan. Cover and cook gently, stirring every so often, for 15-20 minutes, until the apples start to breakdown. Once the pork is cooked, pour about 3.5 floz of the cooking juices into the apples and stir.
To make the apple crisps, core the apples and slice very thin slices through the middle of the apple (1-2mm thick). Dust with cinnamon and lay flat on a baking sheet lined with parchment paper.
Cook in a pre-heated oven at 350°F for 45 minutes to one hour, turning halfway through and removing any crisps that have turned brown. Continue cooking until the apples have dried out and are light golden.