If frozen, thaw in the refrigerator.
When thawed, remove the neck and wings with a sharp knife.
Use paper towels to dry the skin, leave overnight uncovered to allow the skin to finish drying.
Remove any visible excess fat and any liquid in the cavity.
Set up the grill for using a rotisserie rack.
Preheat the grill to high.
Rub salt and pepper into the skin, sprinkle into the cavity.
Chop the lemon in slices.
Mince the spring onions and lemongrass.
Stuff the cavity with the lemon slices and the other seasonings.
Place duck on the rotisserie, then put a drip pan underneath to catch the fat.
Turn grill down to low heat and close lid.
Cook about 20 minutes per pound.
Duck is ready when the juices are clear and the skin is crispy and brown.
Remove from the grill and allow to rest for about 15 minutes before slicing or removing the legs and breasts.
Serve immediately.