Preheat oven to 450°F.
Coarsely crush peppercorns and 2 teaspoons kosher salt with mortar and pestle, then add garlic and mash until a paste forms. Stir in unsalted butter.
If pork loin has been tied, discard strings. Put loin, fat side down, on a cutting board lined with plastic wrap. Butterfly pork in a spiral cut: Find beginning of a flap on 1 long side of loin (where bone was removed). Starting at inside edge of flap, make a long cut lengthwise down side of loin with a very sharp boning or paring knife, stopping 1 inch from bottom (this is beginning of spiral). Turn knife parallel to bottom of loin and begin to cut your way inward (parallel to bottom), keeping thickness of meat as even as possible, using your other hand to gently lift and pull top portion of meat away from knife, until loin is 1 long flat piece of meat.
Cover pork with a sheet of plastic wrap and pound to 1/2 inch thick with a smooth meat pounder or rolling pin. Remove plastic wrap and spread 1 tablespoon truffle butter over pork. Top with half of mortadella, slightly overlapping slices. Spread 1 tablespoon truffle butter over mortadella, then top with remaining mortadella and spread with remaining tablespoon truffle butter. Beginning with end that was interior of loin, roll up loin tightly and arrange, seam side down (fat side up), on cutting board. If fat layer is 1/4 inch thick, make very close crosswise cuts in it (about 1/8 inch apart; do not cut through to meat), then tie with kitchen string at 1-inch intervals. Rub roast all over with peppercorn butter, covering fat layer well.
Put pork, fat side up, on oiled rack in roasting pan and roast in middle of oven 20 minutes.
While pork is roasting, peel and halve potatoes. Parboil potatoes in a 5- to 6-quart pot of boiling salted water 5 minutes. Drain in a colander 5 minutes, then toss with oil, rosemary, remaining 1 1/2 teaspoons kosher salt, and pepper in a large bowl.
Remove pork from oven and reduce oven temperature to 325°F. Add potatoes to roasting pan, turning them in pan juices to coat, then roast pork with potatoes until thermometer inserted diagonally 2 inches into meat registers 155°F, 45 to 55 minutes. Transfer pork to a platter and let stand 25 minutes.
Increase oven temperature to 450°F and remove rack from roasting pan. Spread potatoes out in pan and roast in middle of oven, stirring every 5 minutes, until golden brown, about 20 minutes more. Transfer to a serving bowl and keep warm.
Skim as much fat as possible from pan juices. Straddle roasting pan across 2 burners, then add broth and deglaze pan by boiling over high heat, stirring and scraping up brown bits, 1 minute. Stir together water and cornstarch, then add to broth mixture and boil, whisking, 1 minute. Remove from heat and whisk in truffle butter.
Discard string, then slice pork and serve with sauce and potatoes.