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1
Preheat the oven to 350F.
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2
With a sharp knife score the skin and fat of the ham deeply in a diamond pattern.
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3
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.
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4
Put the onion in the middle of a roasting pan, arrange a rack on top of it, and put the ham on the rack.
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5
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.
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6
Transfer the ham to a carving board, pull off the cracklings (crisp pieces of skin), reserving them, and remove and discard the remaining fat.
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7
Slice the meat thin across the grain, arrange it on a platter, and keep it warm with the reserved cracklings, covered.
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8
Transfer 2 tablespoons of the fat from the roasting pan to a saucepan and pour off the remaining fat from the pan juices.
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9
Add the water to the roasting pan and deglaze the pan over high heat, scraping up the brown bits.
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10
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.
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11
Whisk in the orange juice, the vinegar, and the sugar, simmer the gravy, whisking, for 5 minutes, and season it with salt and pepper.
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12
Serve the pork with the gravy and garnish it with the parsley and the zest.
