Wash, peel and dice the parsnips and sweet potatoes. Cook for 15 to 20 minutes in lightly salted water until very soft. Drain the cooked vegetables well and mash them very finely with a potato masher.
Add the cream (I recommend soy cream, as it emphasizes the sweetness of parsnips and sweet potatoes) and season with salt, pepper and a little nutmeg as desired. Mix the whole thing well and mash it finely again with a fork or puree with a hand blender.