1) Preheat the oven to 190C/Gas 5.
2) Melt the butter in a large skillet over medium-high heat. Add the brown sugar, cinnamon and diced apples and toss to coat. Saute for 3 minutes, without stirring, to let the apples caramelise on one side. Toss and saute for another 3 minutes. Remove from the heat and cool completely.
3) Butterfly the fillet by cutting the pork lengthwise down the centre to within 1 1/3cm of the other side. Flatten with a meat mallet.
4) When the caramel apples have cooled, spread the mixture down the centre flap of pork. Sprinkle with salt and pepper, to taste. Bring the two sides of the fillet up around the filling to meet. Use butcher string and tie around the roll at 2 1/2cm intervals.
5) Season the stuffed and tied pork fillet with salt and pepper. Sear the fillet in vegetable oil in a large Dutch oven over medium-high heat, turning to get all of the sides. Pour the apple cider over the pork. Roast uncovered for about 1 hour and 20 minutes or until an instant-read thermometer inserted into the centre of the roast registers 68C.
Remove from the oven and let rest, tented with foil, for 15 minutes before slicing. Serve with the pan juices.
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