The creaminess in this comforting potato soup comes from the rice milk and the starchiness of the potatoes themselves.
Add butter, onion, carrot, parsnip, celery root, garlic and parsley to a saucepan and cook over medium-high heat, stirring frequently, for about 10 minutes. Add potatoes and broth, cover and bring to a boil. Then reduce heat to medium low, add marjoram, and simmer for 30 minutes.
Remove marjoram from the saucepan and discard. Remove soup from the heat. With an immersion blender or working carefully in batches in a jar blender, blend the soup until smooth. Return soup to the saucepan and stir in rice milk, nutmeg, salt and black pepper. Heat soup over medium heat for about 10 minutes, then serve.