To prepare kale, cut away stalks and thick vein along center of the leaves. Discard any leaves that are bruised or yellow. Fill the sink with water and a palmful of kosher salt (the salt helps remove any impurities) and wash the kale thoroughly to remove any grit. Repeat two or three times or until there is no grit left on the bottom of the sink; drain. Stack the leaves a few at a time; cut crosswise into ribbons.
Heat oil in a large, deep pot over medium heat. Add the onion and garlic, stir to coat, then add the ham hocks and bay leaves; cook about 10 minutes, or until the onions are softened. Pack in the greens, pushing them down into the pot; then add the stock, vinegar, sugar, and red pepper flakes. Bring to a rolling boil and cook for 10 minutes, until greens start to wilt, then turn the greens over with a wooden spoon and lower the heat until the liquid is simmering. Cover; cook for 45 minutes. Taste the broth and add kosher salt as need. Cook, covered, for an additional 10 minutes. Remove bay leaves. Serve hot. Makes 8 to 10 servings plus leftovers.