Preheat oven to 350F.
In a large oven-safe skillet (with lid), heat canola oil over medium high heat. Sear beef, turning as needed until well-browned; move to plate. To pan, add onion, carrot, celery and garlic. Cook, stirring, for 1 minute. Add sherry (or wine) and beef stock, scraping bottom of pan. Add tomato paste, Worcestershire sauce, soy sauce, horseradish, rosemary, thyme, salt and pepper. Bring to a simmer. Stir in cornstarch mixture. Place lid onto skillet and set into oven until meat is fork-tender, about 2 hours.
Meanwhile, in medium pot, cover potatoes with water. Bring to a boil and cook until potatoes are fork-tender, about 20 minutes. Drain. Return to pot, add butter and salt and mash until smooth. Stir in parmesan cheese. Set aside, covered, until use.
Use forks to lightly shred beef. (If beef mixture is too dry, add warm water, ¼ cup at a time, until desired consistency). Top with potato mixture, using spoon to smooth top and create peaks and valleys in topping. Set under broiler until lightly browned, about 2-5 minutes. Serve.