1. Set oven at 400ºF. Season the pork loin liberally with salt and pepper. Heat an ovenproof skillet and oil (cast iron is perfect here) over medium heat, and when the oil is shimmering, sear the pork loin fat side down until browned and beginning to crisp – about 4 minutes. Flip the loin and place the pan in the preheated oven for 20 minutes.
2. While the loin is roasting toss apples, garlic and rosemary together. Remove the roast from the oven and carefully take the roast out of the pan and place it on a plate. Toss the apple mixture in the pan and coat with the fat and juices that have accumulated. Push apples to the sides of the pan and place the roast in the centre. Put the roast back in the oven until the internal temperature is 145ºF, about another 10 to 25 minutes. While the roast is finishing mix together apple cider vinegar, sugar and mustard and set aside.
3. Once the roast is cooked, remove from the pan along with the apples. Place the pan and any accumulated fat and juices on a burner set at medium heat. Add the vinegar mixture and bring to a simmer stirring constantly and scraping any bits remaining on the bottom of the pan so that they incorporate into the sauce. Once the mixture has thickened remove from heat and season to taste with salt and pepper.
4. Slice the pork loin and serve it with the roasted apples and garlic with the sauce alongside.