Let the roast sit at room temperature for 30 minutes. Position a rack in the middle of the oven and heat the oven to 400°F. Put a roasting rack in a roasting pan or in a heavy-duty rimmed baking sheet. (Line the pan with foil for easier cleanup, if you like.)
Season the pork liberally with salt and pepper on all sides. Turn on the exhaust fan. Heat the oil in a 12-inch skillet over medium-high heat. When the pan is hot, sear the rack of pork until browned on all sides, including the ends, 2 to 3 min. per side. Transfer the meat to the roasting pan, meaty side up. Set aside to cool while you prepare the crust.
Pour off all the fat in the sauté pan, return the pan to medium-high heat, and add the butter. When it stops bubbling, add the onion and reduce the heat to medium low. Cook the onion, stirring occasionally, until soft and medium brown, 10 to 15 min. Add the port, 1/2 cup of the balsamic vinegar, the dried cranberries, chile flakes, and allspice. Raise the heat to medium high and boil until the liquid has almost completely evaporated, about another 10 minutes.
Transfer the cranberry mixture to a food processor and pulse 12 to 15 times to create a slightly chunky paste. Scrape it into a small bowl and fold in the walnuts and parsley. Season with salt and pepper to taste.
Reserve 3 Tbs. of the cranberry mixture in a dish to make a sauce later. Pat the remaining cranberry mixture onto the top and sides of the pork rack.
Roast the pork for 30 min. and then tent a sheet of aluminum foil over the roast for the remaining cooking time to keep the crust from overbrowning. Continue to roast until an instant-read thermometer inserted into the thickest part of the meat reads 140°F, about another 30 min.
Meanwhile, in a medium saucepan over medium-high heat, bring the chicken or beef broth to a boil. Add the remaining 2 Tbs. balsamic vinegar and boil until the mixture is reduced by half, about 10 min. Lower the heat to medium and whisk in the reserved cranberry-walnut crust paste. Season with salt and pepper to taste.
Let the meat rest for 10 min. (it will continue to cook as it rests) before carving into chops and serving with the sauce.
A full-bodied, spicy Shiraz from Australia is a good choice to match the rich flavors of the port, as well as the tannins of the walnuts, in the crust and sauce.