Rinse fillets in cold water; pat dry with paper towels.
Sprinkle flour evenly over fish; shake off any excess.
Heat a large nonstick skillet coated with cooking spray over medium-high heat; add oil. Add fish; cook 4 minutes on each side or until fish flakes easily when tested with a fork. Remove fish; set aside and keep warm.
Add wine to pan, scraping pan to loosen browned bits. Add onions and dill; cook 3 minutes, stirring frequently, or until liquid is reduced to about 1/3 cup. Add tomato, lemon rind, and lemon juice; cook 1 minute or until slightly thick. Remove pan from heat; add butter and salt, stirring until butter melts. Spoon sauce over fish.