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.
2. Mash
Remove the hard center of the yuca (look like nerves). Mash until the mixture is very smooth and there aren't any lumps.
3. Add cream
Incorportate heavy cream and onion powder and mix well. Taste and season with salt to taste if you find it necessary.