Good-quality mortadella can be found at butcher shops, at Italian markets, and the deli section of many supermarkets.
2 tablespoons whole black peppercorns
3 teaspoons kosher salt
5 garlic cloves
2 tablespoons unsalted butter, softened
1 (4-lb) center-cut boneless pork loin roast (4 to 5 inches in diameter), trimmed.
1/2 lb thinly sliced mortadella
1 tspn chopped fresh rosemary
1/4 teaspoon black pepper
For sauce
3/4 cup chicken broth
1/4 cup water
1 1/2 teaspoons cornstarch
1 tablespoon butter
Preheat oven to 450°F.
Coarsely crush peppercorns and 2 teaspoons kosher salt, then add garlic and mash until a paste forms. Stir in unsalted butter and rosemary.
If pork loin has been tied, discard strings. Put loin 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.
Starting at inside edge of flap, make a long cut lengthwise down side of loin with a very sharp 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 (or 2) long flat piece(s) of meat.
Cover pork with a sheet of plastic wrap and pound to 1/2 inch thick with a smooth rolling pin. Remove plastic wrap and spread 1 tablespoon butter over pork. Top with half of mortadella, slightly overlapping slices. Spread 1 tablespoon butter over mortadella, then top with remaining mortadella and spread with remaining tablespoon butter. Beginning with end that was interior of loin, roll up loin tightly and arrange, seam side down (fat side up), on cutting board. Tie with kitchen string at 1-inch intervals. Rub roast all over with peppercorn/rosemary butter.
Put pork on oiled rack in roasting pan and roast in middle of oven 20 minutes.
Reduce oven temperature to 325°F. and roast pork until thermometer inserted diagonally into meat registers 155°F, 65 to 85 minutes. Transfer pork to a platter and let stand 25 minutes.
Make sauce:
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 butter.
Discard string, then slice pork and serve with sauce and roast potatoes.
