Preheat oven to 300F.
Thaw the duck and place on a rack in a roasting pan.
Mix the mustard and honey in a small bowl. Smear it all over the duck.
Halve the orange or lemon and place in the cavity.
Cover the duck loosely with aluminum foil.Bake the duck for 4 hours, until done. Meanwhile, mix the salt and water.
Remove the duck from the oven, drain excess fat, and remove the aluminum foil.
Crank up the oven heat to 400F. Brush the salt water all over the duck. Return the duck to the hot oven and roast for an additional 15 minutes, until skin is caramelized and crispy.