Cut the meat into cubes. Make sure they are all about the same size.
Place the meat in a stockpot and pour in cold water until the meat is just covered. Bring to a boil and simmer for 5 minutes. Drain and rinse the pulp in a colander with cold water.
Peel the onion and add the cloves. Cut the carrot into slices and the celery into pieces.
Return the meat to the pan. Add the onion, carrot, celery, garlic and herbs.
Pour in the veal stock and very generously with salt and pepper. Bring to a boil. Skim the surface if necessary.
Put the lid on the pan, lower the heat and let everything simmer for an hour and a half.
Cut the mushrooms into thick slices and fry them briefly in a knob of butter until they are just cooked.
Drain the meat after cooking. Note: place another pan/bowl under the colander, because you have to collect the moisture. You will need some of it to make the sauce. Remove the vegetables and herbs. Transfer the meat to a plate and cover with aluminum foil to keep warm.
Pour the stock into a measuring cup. You need 500 ml to make the sauce. (tip: make sure you keep some extra stock. If your sauce gets too thick, you can thin it later with the extra stock)
Melt the butter in a pan over low heat. Add the flour and stir well for 2 minutes to get a nice roux. Then, while you keep stirring, gradually add the stock. Let it cook for a few minutes to get a nice thick sauce. Season with salt and pepper.
Return the meat to the pan. Stir in the crème frache, egg yolk and lemon juice in a small bowl. Add the egg mixture to the stew and stir well. Heat the mixture for a few minutes. Add the mushrooms and pour in the sauce. Heat everything on low heat. Taste well and, if necessary, season with some extra salt, pepper and/or lemon juice. If you find the sauce too thick, you can thin it out by adding some extra stock. PLEASE NOTE: the sauce should no longer boil, because then the sauce will curdle.