Rinse the duck and thoroughly pat dry. Using a sharp knife, score skin in a crisscross pattern.
In a shallow bowl, combine together the marinade ingredients. Add the duck breasts, skin side up and spoon the marinade over the skin. Set aside at room temperature for 30 minutes.
Preheat Convection to 200°C.
Place the wire rack on the enamel tray. Put the duck breasts skin side up on the wire rack and pat the skin dry with paper towel. When the oven is read, cook on Convection for 8 minutes. Then cook on Grill for 5 minutes. Remove from oven, allow duck to rest on the wire rack for 5 minutes before slicing.
Place the gua bao on a microwave safe plate. Put on the glass dish of the oven and microwave at 600W for 90 seconds.
To assemble, open a gua bao, spread with hoisin sauce, top with 2 batons of cucumber, 1 baton of spring onion, two slices of duck and a few coriander leaves.