Cook vegetables in separate saucepans of boiling salted water until just tender. Drain.
Melt the butter in a saucepan; add the flour and cook, stirring with a wooden spoon, until smooth. Pour in the stock and simmer, stirring constantly, until soup is slightly thickened. Skim if necessary. Remove saucepan from heat; mix egg yolks and cream together, and strain into soup, stirring constantly. Simmer gently over a low heat until the yolks thicken, but do not allow the soup to boil again, or it will curdle.
Just before serving: add the chopped parsley or tarragon and