This rich mash is more than just a vehicle for gravy. It's a subtly sweet blend of parsnip, celery root and two kinds of potato, plus plenty of butter, sour cream and nutmeg.
In a 6-quart pot, bring 4 quarts water and 1 tablespoon salt to boiling. Add celery root and parsnips. Return to boiling; reduce heat. Cook, uncovered, for 10 minutes. Add potatoes. Return to boiling; reduce heat and cook until potatoes and root vegetables are tender, 13 to 15 minutes more. Drain vegetables and return to pot. Set over low heat; toss vegetables to dry slightly.
While root vegetables and potato are cooking, heat milk and butter in small saucepan until butter melts; set aside.
Using a potato masher or food mill, mash potatoes and root vegetables until quite smooth. (A few lumps will likely remain.) Add milk mixture to potatoes; stir to combine. Stir in sour cream, kosher salt, pepper, nutmeg and additional milk as needed for desired consistency.