Rinse the veal brain in cold water, remove the membrane and red veins, then braise it in the butter, covered, over low heat, together with the onion.
Add the rice, the mushrooms and the peppercorns, pour in a little of the broth, and braise, covered, stirring carefully, until the rice is tender (about 15-18 minutes).
Sprinkle with the flour, mix in, then pour in the rest of the broth, and bring to a boil.
Cook over low heat for another 5 minutes.
When serving, mix in the egg yolk, the cream and the butter, don't boil it any further.
Serve with dry-toasted (not buttery) bread cubes.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.