Cut the focaccia into small cubes. Dice onion and celery. Mince garlic and rosemary. Add olive oil to a skillet on medium heat. When hot add onion, celery, garlic, rosemary, salt and pepper and cook until soft. Add chicken stock and turn off heat. Cool slightly and mix in the focaccia. Set aside.
Preheat oven to 350 degrees. Filet the pork and pound with a meat mallet to make pork even thickness. Season both sides with salt and pepper. Coat the inside with some of the balsamic glaze. Spread stuffing mixture evenly over the pork. Roll up pork and tie with butchers twine. Place on top of potatoes and glaze with half of balsamic glaze.
Place pork in the oven and cook for 1 hour - 1 hour 15 minutes. Add the rest of the glaze and rotate halfway through cooking. When pork is cooked, remove from pan, scrape the potatoes from the bottom of the pan. Cool pork for 15 minutes before slicing. Serve pork with potatoes.