If you want to make the whipped parsnips with sweet potatoes early for a party, just put them in an oven friendly baking dish with a lid, and they will keep in a low-temperature oven without drying out for quite a while.
Want you mash to be smooth and silky, then you could run it through a food mill. I don’t mind the texture from just a beater, but if you want restaurant-style, that’s the way to get it.
To change up the flavor, I sometimes add about ½ teaspoon of ground coriander; it’s an excellent addition to the sweet potato that will not overwhelm the parsnip flavor.
Why is there garlic in these photos? Because I didn’t save any of my parsnips for the pictures. I forgot to buy extra. Sorry. But I liked the purplish garlic with the gray tablecloth.