Slice onion into roasting-tin, put prepared duck on top, baste with hot fat and roast in a fairly hot oven (375°–400° F., Gas 5–6) until tender. Melt butter, add flour and cook slowly until the roux browns, stir in stock and simmer until required. If a whole duck has been used, remove trussing string, cut duck into small joints and add with the olives to sauce, season and reheat thoroughly. Sieve (or finely chop) the onion, add to the duck. Drain off the fat and add the sediment from the roasting tin to the sauce.
The salmi may be dished on a croûte of fried bread placed in the centre of a hot dish with sauce and olives poured over, if liked.