1. In a medium saucepan, combine the balsamic vinegar with the onion and 2 thyme sprigs and boil over moderately high heat until reduced to 1/3 cup, about 10 minutes. Transfer the mixture to a heatproof cup. Add the stock to the saucepan and boil over moderately high heat until reduced to 1 cup, about 10 minutes. Add the balsamic reduction and boil until the liquid is thickened and reduced to 2/3 cup, about 15 minutes.
3. Place the pork roasts on a large rimmed baking sheet and season generously with salt. Coat the roasts with the glaze and let stand for 30 minutes, brushing occasionally (the glaze will slide off the meat).
4. Preheat the oven to 300°. Set a rack in a large roasting pan. Transfer the roasts to the rack, allowing the glaze to drip back onto the sheet. Top the roasts with the remaining 4 sprigs of thyme and the garlic and rosemary; season with salt. Add 1/2 inch of water to the roasting pan and roast the pork for about 2 1/2 hours, until it is richly glazed and an instant-read thermometer inserted in the thickest part of the roasts registers 145°. Add water to the pan as it dries out and baste the meat with the pan juices from time to time during roasting.
5. Transfer the roasts to a cutting board and cover loosely with foil. Let rest for 20 minutes. Carve the roasts into chops and arrange on a platter. Drizzle with the reserved balsamic sauce and serve.