Hi,
The way I usually make it is "sancochado" which means boiling them in salted water. The same way you would make potatoes, platanos, yucca, etc. As for the onions, I usually cut up a white onion (vidalia's are my fav) and put in white vinegar for a few minutes, with a little bit of salt and pepper. I heat up 1-2 tablespoons of olive oil or corn oil and heat put the onion mixture until the onions are a little bit soft but firm. Delicious on top of the guineito or yucca!