The spinal cords are sold with the brains if you order them in advance. Wash the brains and spinal cords in cold running water until the spots and skins can be removed easily. Put 4 tablespoons vinegar, salt, pepper, bouquet garni and quartered onion into 4 1/4 pints of water. Boil for 15 minutes, then allow to cool a little. When only warm, add the brains and the spinal cords cut into 1 1/2 to 2-inch long sections. Bring to a boil then, over a very low heat, simmer, covered, for 20 to 25 minutes. Take out the brains and allow to cool. Slice the mushrooms thinly, rub with lemon juice, cook for 15 minutes in butter over a low heat without allowing to brown. To prepare the sauce, take 1 to 1 1/4 cups of the sieved court-bouillon, dissolve the cornstarch into a little cold court-bouillon or cold water, pour into the court-bouillon, boil while stirring. Then add the mushrooms and their cooking water, the egg yolks mixed into the cream with a few drops of lemon juice. Adjust the seasoning. Cut the brains into 1/2-inch thick slices, cover with the sauce, heat gently, serve sprinkled with chopped parsley.
