Bring a large pot of water to boil; add potatoes, garlic and celery root and boil until soft, about 20 to 25 minutes. Drain, and place in a large bowl.
Combine potatoes with butter, milk, salt, and pepper. Mix with an electric mixer or potato masher to your desired consistency. Sprinkle with sesame seeds. Cover and keep warm until ready to serve.
In a large bowl, mix together the ground beef, egg, bread crumbs, pepper, salt, onion powder, garlic powder, and Worcestershire sauce. Form into 8 balls, and flatten into patties.
Heat the oil in a large skillet over medium heat. Fry the patties, onion, and mushrooms in the oil until patties are nicely browned, about 4 minutes per side. Remove the beef patties to a plate, and keep warm.
Sprinkle flour over the mushroom and onion drippings in the skillet. Stir in flour with a fork, scraping bits of beef off of the bottom as you stir. Gradually mix in the beef broth and season with salt.
Simmer and stir over medium-low heat for about 5 minutes, until the gravy thickens. Turn heat to low, return patties to the gravy, cover, and simmer for another 15 minutes.
Serve Salisbury steak patties on top of a bed of garlic mashed potatoes. Make sure to include and smother the patties and potatoes with a generous amount of gravy.