Tie cooking twine around the pork loin in a roast style and place the meat in a large roasting pan.
Combine the ingredients for the sweet & sour marinade, pour it over the pork loin in the roasting pan, and let marinate for 30 minutes.
Heat a large pan over medium heat and sear the marinated pork loin on each side. Pour the sweet & sour marinade over the pork loin and let simmer over low heat for about 20 minutes, frequently spooning the marinade over the pork loin so that it doesn't dry out.
Place the pork loin and marinade back in the roasting pan, transfer the pan to a preheated oven, and cook at 400°F for 20 minutes, making sure to spoon the marinade over the meat every 5 minutes.
For the rosemary potatoes, cut the potatoes in half and fry them in hot oil, together with the rosemary and garlic, until golden brown. Blot the excess oil from the potatoes and season with salt and pepper.
Puree the remaining sweet & sour marinade into a sauce and serve with the pork loin and rosemary potatoes.
