How to Make It
In a large bowl, combine 1/2 cup of the olive oil with the thyme, rosemary, garlic, bay leaf and 2 tablespoons of the saba. Add the pork and turn to coat with the marinade. Cover and refrigerate overnight, turning a few times.
Preheat the oven to 350°. In a large ovenproof skillet, heat the remaining 2 tablespoons of olive oil. Remove the pork from the marinade; reserve the marinade. Season the pork all over with salt and pepper, add to the skillet and cook over moderate heat until browned all over, about 2 minutes per side. Transfer to a plate.
Add the red wine to the skillet and bring to a boil over high heat, scraping up any browned bits from the bottom of the skillet. Add the vegetable stock and the reserved marinade and bring to a boil. Add the pork, fat side up, and roast in the oven for about 50 minutes, or until an instant-read thermometer inserted in the center of the loin registers 140°.
Transfer the pork to a rimmed baking sheet and let rest for 10 minutes. Strain the liquid from the skillet into a medium saucepan and bring to a simmer over moderate heat. Add 1/4 cup of the saba and stir in the cornstarch slurry. Simmer the sauce, stirring, until thickened, about 3 minutes. Season with salt and pepper.
Preheat the broiler. Brush the top of the roast with 2 tablespoons of the saba and broil for about 1 minute, or until glazed. Turn the roast, brush with the remaining 2 tablespoons of saba and broil for 1 minute. Let the pork rest for 10 minutes. Carve the roast and serve with the sauce.