Bisque of Prawns or Shrimps

Boil three dozen prawns twenty minutes in salted water to cover them.
Meanwhile in two small tablespoonfuls of butter, fry an onion and a
carrot sliced, and a small piece of salt pork chopped. Take the prawns
out of the boiling water and add to it the fried mixture with salt,
pepper, a bunch of sweet herbs and one-half the prawns added again.
Simmer one hour. Pound the shells of the prawns in a mortar with a
little butter, to form a smooth paste. Stir this into the soup and
boil twenty minutes. Strain through a sieve. Add one quart of milk and
one teaspoonful of cornstarch stirred into a little of the cold milk.
Let it boil up, and serve. It should be as thick as rich cream.