1. Thaw fish, if frozen. Rinse fish and pat dry with paper towels; set aside.
For the sauce, in a small bowl stir together mayonnaise dressing, pickle, yogurt, parsley, onion, and tarragon or mustard. Let stand at room temperature while preparing fish.
Spray the unheated rack of a broiler pan with nonstick cooking spray. Arrange fish on rack and brush both sides of fillets lightly with oil.
Broil fish 4 inches from the heat until fish flakes easily when tested with a fork. Allow 4 to 6 minutes per 1/2-inch thickness of fish. If fish is 1 inch or more thick, turn it over halfway through broiling. Serve fish with sauce. Makes 4 servings.