Preheat the oven to 350F. With a sharp knife score the skin and fat of the ham deeply in a diamond pattern. In a blender puree the garlic with the orange juice, the oil, the vinegar, the salt, the pepper, and the oregano and rub the mixture all over the ham. Put the onion in the middle of a roasting pan, arrange a rack on top of it, and put the ham on the rack. Roast the ham in the oven for 4 hours, or until a meat thermometer registers 170F., and let it stand in the pan at room temperature for 15 minutes. Transfer the ham to a carving board, pull off the cracklings (crisp pieces of skin), reserving them, and remove and discard the remaining fat. Slice the meat thin across the grain, arrange it on a platter, and keep it warm with the reserved cracklings, covered.

Make the gravy:

Transfer 2 tablespoons of the fat from the roasting pan to a saucepan and pour off the remaining fat from the pan juices. Add the water to the roasting pan and deglaze the pan over high heat, scraping up the brown bits. Add the flour to the fat in the saucepan, cook the roux over low heat, whisking, for 2 minutes, and strain the mixture from the roasting pan through a sieve into the roux, pressing hard on the solids. Whisk in the orange juice, the vinegar, and the sugar, simmer the gravy, whisking, for 5 minutes, and season it with salt and pepper.

Serve the pork with the gravy and garnish it with the parsley and the zest.