Wash potatoes and celeriac. Use a peeler to remove the skins from the potato. Use a knife to cut off the skin of the celeriac. Chop your roots into 1-inch pieces.
Place prepared potato and celeriac in a saucepan, and cover with at least 2-inches of water. Bring to a boil and simmer about 15 minutes until the vegetables are soft. To check, use a knife to pierce a potato. If it comes out easily when you lift the potato with the knife, or the potato breaks apart, the potato is soft.
Drain off the water, retaining the roots in the saucepan. Add whole milk, butter, and salt. Mash with a potato masher until smooth. Serve immediately, with, or without, Mushroom Gravy.
Method for vegan mushroom gravy
Heat oil in a medium saucepan on medium heat. Add onion and portobello mushrooms. Sauté until medium to dark brown, about 10 minutes.
Add flour and stir with a wooden spoon to combine. Allow the flour to cook for a couple minutes. Add vegetable stock a little at a time, stirring in between each addition until smooth. Bring to a boil, and then reduce to a simmer. Add the seasonings, and continue to cook until a desired consistency is achieved. Taste and adjust final seasoning.
Serve mashed potatoes with celeriac right away. If you need to keep it warm, you can keep it in a covered dish in the oven. You can also keep the covered mashed potato inside a larger pot on the stove with a couple inches of simmering hot water, like a double boiler.Serve vegan mushroom gravy hot just as it is, or pass the gravy through a sieve if you want to remove the textured pieces and create a silky-smooth sauce.
MAKE THIS VEGAN
Use olive oil or coconut oil instead of butter. Use plant-based milk instead of whole milk.
MAKE THIS GLUTEN-FREE
Use rice flour instead of all-purpose. It substitutes measure for measure, according to Serious Eats.