Stir together flour, sugar and mustard. Add egg yolk and beat. Beat in cream.

In a saucepan, heat vinegar until steaming, but do not boil.

Combine all ingredients together in the pan but do not allow to boil. Simmer for about 15 minutes over lowest heat or until thickened, stirring occasionally.

This sauce is good over ham in sandwiches, with liverwurst or basted on chicken during cooking. Also good with boiled dinners.