Put the meat to boil in cold, slightly salted water (the water should just cover the meat) on low heat. After it boils make sure you remove the froth that forms on the surface of the water; do that 2-3 times.
Add the carrot and the onion (skin removed), for flavour. Let everything boil until the meat is cooked, then put meat aside and reserve the liquid.
In a separate pan melt the butter and sprinkle the flour over it. Combine it very well with a whisk and let it cook for a minute on low heat, making sure that the flour doesn't brown; then add, little by little, the liquid resulted from boiling the meat, until the mixture reaches the consistency of sour cream and does not have any lumps. Keep on whisking!
In the meantime, mix the egg yolks with the sour cream then pour it over the flour mixture. Correct the seasoning and, of course, whisk it some more! :D
Put the meat (and the mushrooms, if using) in the sauce, and allow to cook for 2-3 minutes more, stirring continuously and without boiling it.
Sprinkle with chopped fresh parsley and serve!
If you also use mushrooms, saute them in butter before adding to the white sauce.