Preheat oven to 350 degrees. Salt and pepper fillets and place in shallow oven-proof dish. Cover with white wine and lemon juice. Place in oven and poach in liquid until fish flakes easily with a fork but is still moist, about 15 minutes. Melt 3 tablespoons of butter and saute shrimp and onions until shrimp are pink and onions wilted, about 5 minutes.
In another pan, melt a tablespoon of butter. Stir in flour, stirring a minute or two. Gradually stir in milk or cream. Cook, stirring over medium heat until sauce thickens. Stir 1/2 cup poaching liquid into sauce and continue cooking until sauce comes to a boil. Reduce heat. Add shrimp and onions. Arrange fish on platter. Spoon sauce over. Garnish with parsley and lemon slices. Serves 4.
