Place in saucepan the fish and wine and allow to simmer gently until tender. Season. Remove from pan and keep warm until sauce is made.
Melt butter, add flour to make smooth paste. Add enough wine to the liquid in which fish was cooked to fill 1 cup. Add to butter and flour and cook over direct flame until the mixture thickens. Cool slightly and add egg yolk. Season to taste. Pour sauce over fillets and serve with slices of onion, boiled potatoes and garnish with parsley and lemon.
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