1. Place 1 inch of water and the vinegar into a skillet over high heat and bring to a boil.
2. Add the salmon, reduce the heat to low, cover the skillet and cook 10 minutes.
3. Blend the dill, scallions, mayonnaise, sour cream, horseradish and mustard in the blender until the scallions are chopped fine.
4. Remove the salmon to a platter and top with the horseradish sauce.