1. Set oven at 200 ºC. Wash duck well, inside and out.
Trim off any loose fat around the neck. Cut off the wings at the 'elbow' and use to make stock. Remove the fat gland at the base of the tail. Rub the area with a little salt and lemon juice.
Pour boiling water over and pat dry with paper towels. Prick skin thoroughly (this allows the fat to drip away during roasting) with a sharp, two-pronged fork or skewer and rub in ginger, garlic, salt and pepper.
To shape the duck for roasting, skewer the wings and neck flap to the body with thin metal skewers.
2. Place on a rack in a roasting pan containing 500-750 ml (2-3cups) water. Cover duck with a foil tent and roast for 30 minutes.
3. Remove foil and continue roasting for about 1 hour longer. Salt the liver and roast it with the duck for the last 5 minutes.
To test for doneness, prick the duck with a skewer or two-pronged fork. If the juices that run out are slightly rosy, the duck is roasted medium rare; if they are pale yellow, the duck is well done. It's best not to overroast - the skin should be crisp, but the flesh should be succulent.
4. Remove duck from roasting pan. Allow to rest for about 10 minutes before carving. To carve: Remove drumsticks and wings, turn duck onto its side and carve thin slices from breast. Repeat on other side.
SAUCE: Pour off fat from pan and add stock, puréed mango, orange juice, orange peel and liqueur. Stir with a wooden spoon, scraping in brown bits. Strain through a strainer into pan, add butter (this gives it gloss and a touch of richness) and cook, uncovered, until sauce thickens slightly (reduced by a quarter). Serve in a heated sauce boat.
5. Slice liver thinly and serve with duck, sauce and vegetables.