Preheat an oven to 400 F.
Let the pork roast come to room temperature for at least 30 minutes before cooking. Salt the exterior of the meat generously.
In a medium-sized mixing bowl, toss the quartered onions and apples with the olive oil, and salt lightly. Place the pork roast in the roasting pan, arrange the apples and onions around it.
While the pork is roasting, place the cranberry relish in the work bowl of a food processor and purée until smooth and liquid, about 1 minute.
Roast for 30 minutes, reduce the oven heat to 350 F and continue to roast the pork until it reaches 135 F. Remove the roast from the oven and brush the surface with the cranberry glaze. Place the pork back into the oven. Continue cooking until the internal temperature has reached 155 F. Remove the roast from the oven, cover lightly with aluminum foil and let rest for at least 20 minutes before carving.