Prep duck breast by patting dry and leave uncovered, skin side up, in the fridge overnight.
Heat wok with a little vegetable oil on low to medium heat.
Place duck breast skin side down in the wok.
Spoon some plum sauce on the duck.
Cover with a lid and cook for about 10 minutes.
Turn temperature to low, flip duck breast, and cook for another 4 minutes with lid back on.
Let duck cool, then slice.
Assemble in steamed buns with cucumber and green onions.
Add plum sauce or hoisin sauce.(optional)