Place the rinsed yautía in a pot, and add sufficient water to cover it plus a couple of inches [5 cm]. Add ¾ tablespoon of the salt to the water. Boil until they are fork-soft. Once boiled remove from the water and discard the water.
Mash until the mixture is very smooth and there aren't any lumps (a potato ricer or food mill is perfect for it).
Incorportate butter, and then mix in the milk. Incorporate the garlic powder. Taste and season with salt to taste if you find it necessary.
Cover with the sauteed onion, and serve.