Combine 4 quarts cold water, 1 cup salt, and the garlic, onion, peppercorns, coriander, bay leaves, and parsley in a container big enough to hold the ham, and stir until the salt is dissolved. Submerge the ham in the liquid and refrigerate for at least 24 hours and up to 3 days. (Alternatively, if you can't fit the ham in your refrigerator, brine it directly in a clean cooler, just adding a little extra salt and enough ice to keep the ham cold, draining a little brine and adding more ice as needed.)

Preheat the oven to 350˚F

Drain off the brine, discarding all the seasonings, and pat the ham dry. Put a paper towel on top of a cutting board and set the ham on top of that so that it doesn't slip. With a sharp knife, score the skin with incisions that run the length of the ham and are about 1/2 inch apart, and then again the other way to form a crosshatch pattern. The incisions should just barely reach into the fat under the skin; do not cut into the meat itself, in order to help the crackling skin stay in one piece once it is crispy. Allow the ham to come to room temperature.

Lightly oil and salt the meat, rubbing it in on all surfaces. Put the ham on a rack or on a few rolled-uo sheets of aluminum foil in a large heavy roasting pan and put it in the oven. A 15-pound roast will take almost 4 hours total, while and 18-pounder will take as long as 5—about 15 minutes per pound. About 2½ hours in, when the temperature of the ham hits about 130˚F, raise the oven temperature to 425˚F to crisp the skin (cover any areas that start to get too dark with a piece of foil). When the meat thermometer reads 145˚ to 150˚, remove the roast and allow it to rest, loosely tented with foil, for an hour or so before carving.