Place fish fillets in baking pan coated with nonstick cooking spray.
Mix together lemon juice, onion, and 2 tablespoons melted margarine. Pour mixture over fish. Bake uncovered or until fish flakes easily with fork, about 15 minutes.
While fish is baking, prepare creole sauce; saute green pepper in remaining margarine. Add tomatoes and pepper. Stir in flour. Simmer until mixture is heated.