1. Preheat oven to 180°C. With a sharp knife score the skin and fat of the pork leg deeply in a diamond pattern.
2. Whisk together garlic, orange juice, oil, vinegar, salt, pepper and oregano.
3. Rub mixture into the meat. Put the onion in the middle of a roasting pan,
arrange a rack on top of it, and put the
pork on the rack.
4. Roast in the oven for 4 hours, then let stand in the pan at room temperature for 15 minutes.
5. Transfer the pork roast to a carving board, pull off the crackling (crisp pieces of skin), reserving it; remove and discard the remaining fat.
6. Slice the meat thinly across the grain, arrange it on a platter, cover and keep warm with the reserved crackling.
7. For the gravy: Transfer 2 tablespoons of fat from roasting pan to a saucepan and pour off remaining fat from pan juices. Add water to roasting pan and
de-glaze pan over high heat, scraping up the brown bits.
8. Add flour to fat in saucepan, cook roux over low heat, whisking for 2
minutes, strain mixture from roasting
pan through a sieve into roux, pressing
hard on the solids.
9. Whisk in orange juice, vinegar and sugar; simmer gravy, whisking for 5
minutes, season with salt and pepper.
10. Serve pork roast with roast potatoes and gravy.