Preheat the oven to 350ºF.
Remove the duck from the packaging and rinse it thoroughly under water.
Pat the duck dry and place it in a foil-lined roasting pan.
Insert the sliced onion and garlic cloves into the duck.
Rub the exterior of the duck with the butter and salt it thoroughly. Fold the foil over the duck to cover it.
Place the roasting pan in the oven and cook the duck for 2 hours and 15 minutes.
Remove the pan from the oven and uncover the duck.
Increase the oven to 400ºF.
Return the duck, uncovered, to the oven and allow it to cook until golden brown and the skin is crispy to the touch (the cooking time will vary according to the type of oven & oven rack placement).
Remove the crispy duck from the oven and allow it to rest for 10 minutes before carving.
Heat a mu shu wrapper in a non-stick skillet over medium heat just until warm.
Spread a serving of Hoisin sauce on the warmed wrapper, then add the scallions, cucumber and duck. Fold like a burrito and serve.
Helpful Hint: The duck does not yield as much meat as you may anticipate, so I would suggest one duck per two or three people.
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