Combine celery root, onion, apple, sage and chicken broth in a 4-quart saucepan, Bring to a boil, reduce heat and simmer for 15 minutes, until celery root is tender. Remove sage leaves and drain stock, reserving liquid. Place cooked mixture into a food processor with salt and add 1/2 cup reserved liquid, then more liquid by the tablespoon to achieve desired consistency. Puree for 30 seconds until smooth. Reserve (mixture will stay warm in food processor bowl for about 30 minutes).
Make Pork: Preheat broiler. Pat pork loin dry with paper towels. Combine sugar, celery seed, sage, salt and pepper in a small bowl. Rub spice mixture all over pork loin. Place on broiling tray and broil approximately 7 to 8 minutes per side, until meat is cooked but still pinkish in center. Remove and let rest 5 minutes, then slice into 1/4-inch slices.
Heat oil in a medium-sized saute pan over medium heat, Saute shallots, stirring often, until soft and translucent but not brown about 5 minutes. Add apple cider and cook until all but 2 tablespoons of the cider has been cooked away and shallots are glazed with reduced cider, about 4-5 minutes. Add chicken broth, vinegar and salt and cook an additional 2 minutes. To serve, spoon 1 cup puree onto a plate. Top with 4 slices of pork and 1 1/2 tablespoons cider gravy.