Add the potatoes and celeriac to a large pot. Fill with cold water. Season with salt liberally. Place over high heat and bring to the boil. Cook for 15-20 minutes or until the potato and celeriac can be mashed easily with a fork. Drain. Return the potatoes and celeriac to the pot.
Place the pot back on the stove over low heat. Mash until fairly smooth. Add the butter and milk and continue to mash until creamy and smooth. Add more milk if needed.
