Preheat an oven to 190C. Pat the duck dry with some paper towel and if time, leave uncovered in the fridge overnight.
Rub the duck with the olive oil, season with salt/pepper & stuff the cavity with the rosemary & garlic cloves.
Place the duck onto a rack in a baking tray and roast for 1 1/4 hours.
Simmer the potatoes in lightly salted water until just tender, approx. 15 minutes. Drain through a colander, gently shaking to drain and rough up the outside of the potatoes (this helps get them nice and crispy later). Allow to cool.
Combine the parsley, rosemary, garlic, olives, lemon zest/juice and olive oil in a small bowl, seasoning to taste.
After 1 hour of cooking, remove the duck on its rack from the baking tray and pour the duck fat into a new, large baking dish.
Add the potatoes to this, tossing well & seasoning. Roast for 1 ½ hours, shaking them in the tray throughout cooking until they are golden & crisp.
Place the onion into the duck tray, lay over the duck & scatter the grapes around. Return this to the oven for approx. 45 minutes then let it rest.
Serve the duck, carved, with the potatoes, grapes, onions & gremolata spooned over.
