Cut the yuca into 2-in pieces and peel with a paring knife. Cut the large pieces in half so that all are of uniform size. Bring ar least 2 quarts salted water to a boil. Add the yuca and cook for 20 minutes or until cracks begin to appear in the center (of the yuca) Add 1 c cold water. Cook the yuca for 5-10 more minutes, or until very soft. Meanwhile, make the mojo. Finely chop the garlic, and, in a mortar and pestle, pound it to a paste with the salt and pepper. Stir in the lime juice and correct the seasoning. Just before serving, heat the olive oil in a small frying pan and pour it into the garlic mixture. Stand back it may spatter. Drain the yuca and pull out any fibers. Arrange on a platter or plates and pour the hot mojo on top. Garnish the yuca with thinly sliced onion and serve