Shred onion and ginger root, sprinkle with salt,and let stand 10 minutes.
Combine sugar and vinegar, then pour over onion and ginger. Let stand 30 minutes more. Drain, discarding liquid.
Bone and cut roast duck in 1/4-inch cubes. Put in a large bowl. Peel pineapple, cut in 1/4-inch cubes and add.
Put sesame seeds in a dry skillet; stir over low heat, until golden. Spread out to cool, then crush with a rolling pin. Add to bowl.
Combine powdered mustard, water, and remaining vinegar and sugar, stirring until mustard smells pungent.
Add mustard mixture to bowl, along with drained onion and ginger, oil, and remaining salt. Toss with duck and pineapple to blend well, and serve.Servings: 4