1. Bring first 4 ingredients to a boil in a large skillet. Reduce heat to medium-low, and add fish; cover and simmer 7 minutes or until fish is firm when touched in center. Remove from heat; cover and let stand 3 minutes.
2. While fish cooks, combine sour cream, cucumber, and 1 teaspoon dried dill in a small bowl. Remove salmon and onion from poaching liquid, using a slotted spoon; top with sour cream mixture.