Boil: Place cassava (yuca) and celeriac in a pot, and add sufficient water to cover it plus a couple of inches [5 cm]. Boil until they are very soft, adding ½ tablespoon table salt to the water. Once boiled remove from the water and discard the water.
Mash: Remove the hard center of the yuca (look like nerves). Mash until the mixture is very smooth and there aren't any lumps (a potato ricer or food mill [Amazon affiliate links] is perfect for it).
Add cream: Incorportate heavy cream and onion powder and mix well. Taste and season with salt to taste if you find it necessary.
Serve: Sprinkle with chopped chives before serving.