Melt butter in a saucepan; add vegetables and herbs, and simmer gently for about 5 minutes.
Bring chicken stock to a boil; pour it over the vegetables and allow the soup to simmer gently for about 30 minutes, or until vegetables are quite tender.
Beat egg yolks and cream with a fork until well blended. Remove saucepan from the heat and strain egg and cream mixture into the soup, stirring constantly. Return pan to heat and simmer, stirring constantly, until the yolks thicken, but do not let it come to a boil, or your soup will curdle. Season to taste with salt and freshly ground black pepper. Serve with croûtons.