Sprinkle fillets with pepper.
Heat a large nonstick skillet over medium- high heat. Coat fillets with cooking spray. Add 2 fillets to pan; cook 3 to 4 minutes on each side or until fish flakes easily when tested with a fork. Remove from pan, and keep warm. Repeat procedure with remaining fillets.
Add wine, onion, garlic, and tarragon to pan; bring to a boil. Reduce heat to medium- high, and cook 5 to 7 minutes or until liquid evaporates. Reduce heat to low; add butter, parsley, and salt. Cook 2 minutes or until butter melts.
Spoon sauce over fillets. Serve immediately.