Fill a large pot with water and salt (add garlic and onions if using) and bring to a boil. Place the pork shank in the boiling water, cover, and boil for 1 hour. Make sure that the water covers the whole shank during the entire cooking time; add more water if needed.
After 1 hour, remove the shank from the water; reserve the cooking liquid.
Preheat the oven to 375°F. Make lengthwise cuts in the skin of the pork shank, about 1 inch apart, all the way around. Season well with additional salt. Place the shank in a roasting pan and add 2 cups of the reserved cooking liquid. Place in the oven and roast for 90 minutes.
Increase the oven temperature to 425°F and roast for 20 more minutes to crisp the skin. This dish is traditionally served warm (not hot) on a wooden platter. Best served fresh.