Bring a pan of water to a boil. Add some salt once the water boils and then the celeriac, and after 15 minutes add the potatoes and apples. Cook for another 20 minutes, until everything is tender. Drain well (celeriac especially contains a lot of liquid). Mash everything; I like to first mash the celeriac so that I can drain the liquid some more, and then I proceed with the rest. You can also use a potato ricer.
Season the puree with salt, pepper, and nutmeg. Fold in the butter and parsley and serve, or if you wish to serve it later, let the puree cool completely and keep in the fridge. Use an ice cream scoop to form balls and place them in a buttered oven dish. Cover until ready to reheat.
Sprinkle the potato balls with the breadcrumbs mixed with parsley, salt, and pepper.
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