Step 1

Place fish in a well-greased 13- x 9- x 2-inch baking pan; pour 3 tablespoons melted butter over fish. Combine water and lemon juice; pour over fish. Place onion on top of fish. Cover and bake at 350° for 30 minutes or until fish flakes easily when tested with a fork. Transfer fish to a serving platter, and keep warm. Reserve pan drippings.

Step 2

Melt 2 tablespoons butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add reserved drippings; cook over medium heat, stirring constantly, until thickened and bubbly. Combine yolk and whipping cream; gradually stir into sauce. Stir in salt and pepper. Cook until sauce is thoroughly heated.

Step 3

Pour sauce over fish on platter. Garnish with parsley sprigs and lemon wedges. Serve immediately.