Put on a kettle of water with a good deal of salt in it, and a good many parsley roots; keep it skimmed very clean, and when it boils up throw in your perch or whatever fish you use for the purpose. When sufficiently boiled, take them up and serve them hot. Have ready a pint or more of water, in which parsley roots have been boiled, till it has acquired a very strong flavour, and when the fish are dished throw some of this liquor over them. The Dutch sauce for them is made thus:
To the white wine vinegar add the mace; let it stew gently by the fire, and, when the vinegar is sufficiently flavoured by the mace, put into it the butter. Shake the saucepan now and then, and, when the butter is quite melted, make all exceedingly hot; have ready the yolks beaten up. You must continue beating them while another person gently pours to them the boiling vinegar by degrees, lest they should curdle; and continue stirring them all the while. Set it over a gentle fire, still continuing to stir until it is very hot and of the thickness you desire; then serve it.