Peel (or scrub) and quarter Yukon Gold potatoes.
Peel sweet potatoes and cut into larger pieces than the potatoes (they cook faster). If using peeled chopped parsnips or celeriac, add in a ratio of 1:3 - parsnips to potatoes.
Put potatoes and garlic into a large pot of salted water and bring to a boil.
Add sweet potato (parsnip or celeriac) and boil until vegetables can be easily pierced with a fork but not disintegrating or water-logged.
Remove from the pot and save the garlicky water for soups and gravies.
Melt butter in the pot and add cream (and to start, mustard or horseradish to taste); heat until warm.
Put potatoes, sweet potatoes and garlic back into the pot and mash.
For a smoother mash, rice the vegetables first before stirring into the heated cream and butter. Add crème fraiche, grated cheese if using, and season with salt, pepper and nutmeg. A food processor might seem like a good idea, but don’t do it - the results will be gummy and unpalatable.
Scoop into a baking dish, even the top slightly with the back of the spoon and serve. Or cover well to keep warm in a low oven for up to an hour or so before serving.
Dot with butter and sprinkle with finely chopped chives to serve.
