1. Cut the yuca crosswise into 2 inch pieces and peel with a knife. (Stand the pieces on end and cut off the peel with downward strokes of the knife.) Cut any large pieces lengthwise in half or quarters, so that all pieces are the same size.
2. Bring 2 quarts lightly salted water to a boil in a large pot. Add the yuca and cook for 10 minutes. Add 1 cup cold water. Return the yuca to a boil and cook for 5 minutes. Add another cup of cold water, return to a boil, and cook for 5 minutes, or until the yuca is very soft. (The additions of cold water help tenderize the yuca.)
3. Meanwhile, prepare the mojo. Heat the oil in a small saucepan over medium heat. Add the garlic and fry until just beginning to brown, 1 minute. (Do not let the garlic burn, or the mojo will be bitter.) Add the lime juice, stock, salt, and pepper and boil for 2 minutes, or until the sauce is richly flavored. Add salt to taste.
4. Drain the yuca and scrape out any fibers with a fork. Arrange the yuca on a platter and arrange the onion rings on top. Pour the hot mojo over the onions and yuca. Sprinkle with parsley and serve at once.