Place thawed duck in roasting pan. Prick skin of duck with a fork in several places. Melt 1 cup butter, pour over duck, and roast in oven at 350 degrees 30-35 minutes per pound, plus an extra 15-30 minutes if stuffed.
In a 2-quart saucepan over medium heat cook onion in 1/4 cup butter until tender. In blender, combine remaining ingredients and blend until smooth. Pour mixture into saucepan and simmer over medium heat for 25 minutes, stirring occasionally.
When duck is done, increase temperature to 400 degrees. Pour fat from duck and discard. Brush duck with plum sauce and roast 15 minutes longer, brushing occasionally with sauce. Serve immediately with extra sauce.