Start by preparing the pork loin. Trim off any excess fat and season it generously with salt, pepper, dried oregano, and chopped fresh rosemary. Drizzle white vinegar over the seasoned pork loin and rub the spices into the meat. Allow the pork loin to marinate in the refrigerator overnight for best results.
Preheat your oven to 325°F (160°C). The next day, remove the pork loin from the refrigerator and let it sit at room temperature for about 30 minutes to take the chill off.
Meanwhile, wash the plums thoroughly, halve them, and remove the pits. If the pits are easy to remove, do so; otherwise, they will come out during cooking.
Place the marinated pork loin in a roasting pan with the plums, and cover it with aluminum foil. Roast in the preheated oven for 2 hours.
After 2 hours, remove the pork loin from the oven and carefully take out the plums from the roasting pan. Return the uncovered pork loin to the oven and continue roasting for an additional 30 minutes, or until the pork loin is golden brown and cooked through.
While the pork loin is roasting, prepare the plum sauce. Take the plums and extract the seeds, preferably by passing them through a coarse strainer. Place the plum puree with 1/2 cup of water in a saucepan over medium heat. Add a packet of stevia (or sweetener of choice) to taste.
In a small bowl, mix 1 teaspoon of cornstarch with a little water to create a slurry. Add the slurry to the plum puree in the saucepan, stirring constantly. Cook the sauce for a few minutes until it thickens slightly and becomes glossy. Season with salt and pepper to taste.
Once the pork loin is done, remove it from the oven and let it rest for a few minutes before slicing.