Step 1Heat the oven to 350 degrees.
Step 2Separate and peel the cloves of garlic. Place in a food processor along with the salt and pepper. Process until a paste forms, about 10 seconds. Rub the paste all over the pork, then place the pork in a large roasting pan. Cover the pan tightly with foil and roast until the meat is very tender, about 3 hours.
Step 3Meanwhile, stir together the sour cream , mayonnaise, mustard and horseradish. Chill.
Step 4Bring the beef broth to a boil in a medium saucepan over medium heat. Reduce the heat to medium-low and add the onion, carrot and celery. Simmer until the liquid has reduced by half, about 35 minutes. Strain the broth, discarding the solids, and add to the roasting pork. Replace the foil and continue cooking.
Step 5When the pork is tender, uncover and continue to roast until lightly browned, about 20 minutes. Remove from the pan, reserving the juices. Cover the pork loosely with foil and let stand, about 30 minutes, then shred the meat. Skim as much fat as possible from the juices.
Step 6Cut the baguettes in half lengthwise then dip each half, cut side down, into the juices. Divide the meat among the baguettes. Garnish with pickle spears, olives or cornichons, if desired. Serve with the remaining juices and chilled mustard mixture on the side.