Heat the butter in a large skillet over medium heat. Add the onions and cook for 10 minutes, tossing the onions around every once in a while. Add the sugar and cloves and cook for 15 more minutes over medium-low heat, stirring occasionally, until the onions are soft, brown, and sweet. Keep warm over low heat.
Meanwhile, soak the fish in ice water for 15 to 20 minutes, or place it in a c