Thaw frozen fish.
Place on greased rack in 10 inch skillet.
Add boiling water to cover.
Add quartered onion and 2 teaspoons salt.
Simmer, covered, till fish flakes easily, 5 to 10 minutes.
Carefully remove fish.
Cover; chill well.
Combine remaining ingredients; chill well.
Spoon sour cream mixture over chilled salmon.
Garnish with lemon and lime slices and fresh dillweed, if desired.
Makes 4 servings.