Place an oven rack in the lower third, and preheat oven to 275°F.
Place the roast on a rack standing in a roasting pan. Holding a blowtorch about 1" from the surface, pass the flame over the entire roast until the fat is lightly browned and smoking gently, and the meaty areas are just barely gray.
Season generously with salt and pepper, and place the pan in the oven with the meaty part towards the back. Cook until a thermometer reads 128°F in the center (about 2 hours; if you do not have a leave-in thermometer, begin checking at 90 minutes). Remove from oven and let rest, tented with foil, in a warm place, 30 minutes for medium-rare.
Meanwhile, whisk the cream and vinegar in a very cold bowl until it holds soft peaks. Whisk in horseradish, salt and pepper. (Unused cream will stay good, refrigerated in an airtight container, up to a week.).
Carve the meat off the bones. Cut between the bones and place them on a serving platter. Cut the roast in half through the center, place the cut sides down on a board, and carve into 1/2" thick slices. Arrange on the serving platter, sprinkle with grey salt or sea salt and black pepper, and serve with horseradish cream alongside.