Preheat a large skillet on medium-high heat and when very hot, sear the duck breasts for 2 minutes each side.
Remove from the pan and slice thinly.
Artfully fan out the duck slices on large plates and coat with the dill and mustard sweet and sour sauce. Garnish with green onions and serve with grilled bread and a green salad with beets.
Put the first 6 ingredients in a medium bowl and beat thoroughly until the sugar and dry mustard are completely dissolved.
Gradually drizzle in the olive oil, beating continuously until the sauce is thoroughly emulsified.
Cover tightly and store in the fridge.