1. Place potato in a medium bowl; cover with cold water to 1 inch above potato. Set aside.
2. Melt butter in a large Dutch oven over medium heat. Add onion to pan, and cook 7 minutes or until soft but not browned, stirring occasionally. Add leek and the next 4 ingredients (through celery root) to pan. Place a sheet of aluminum foil directly over vegetable mixture. Cover, reduce heat to low, and cook 15 minutes or until vegetables are tender. Discard foil.
3. Drain potatoes; add to pan. Stir in 2 cups water. Cover and simmer 20 minutes or until vegetables are tender, stirring occasionally.
4. Heat milk in a small, heavy saucepan over medium heat to 180° or until tiny bubbles form around the edges (do not boil). Gradually stir hot milk into vegetable mixture, stirring constantly. Remove from heat. Taste and adjust seasoning, if desired.
5. Preheat broiler.
6. Place bread slices in a single layer on a baking sheet. Broil 1 minute or until toasted. Place 1 bread slice in each of 8 soup bowls; top each bread slice with 1 cheese slice. Ladle 1 1/4 cups soup into each bowl.