Peel potatoes and cut them in half or thirds to make relatively similar-sized potatoes. Then peel garlic and chop. Cut off the ends of the celeriac root then slice and trim off the rest of the outer skin of the root. Chop into approximately 3/4 inch dice. Place in pot and fill with water just to cover the top of vegetables. Bring to a boil and cook until potatoes slide off the knife easily when piercing with a knife and celeriac is tender- approximately 20 or 25 minutes. Reserve 1/4 cup of the cooking water then drain the vegetables. Place potatoes, celeriac, and garlic back into the empty pot heat off, and let dry out for a minute then mash with a potato masher. Add sour cream and warmed soymilk with a little of the cooking water. Stir and combine well. Then add nutmeg and thyme, salt, and pepper. Add more cooking water to reach desired consistency. Top with vegan butter and let melt on the finished potatoes. Serve warm.
Notes
To reheat premade mashed potatoes, place in pot, add a little soymilk and stir over low heat until desired consistency is reached.