Heat butter in a 6-qt. saucepan over medium-high heat; add garlic, onion, and leek, and cook, stirring, until soft, about 3 minutes.
Add potatoes, stock, cream, celery root, horseradish, marjoram, cloves, and bay leaves, and bring to a boil; reduce heat to medium-low, and cook, stirring occasionally, until potatoes are soft, about 30 minutes.
Season with nutmeg, salt, and pepper; remove from heat, and using an immersion blender, regular blender, or food processor, purée soup until smooth and frothy.
To serve, ladle soup into 4 bowls, and garnish with chives, caramelized onions and cheddar cheese.