Cook the Veal:

Place the veal cubes in a large pot and cover with water. Bring to a simmer over medium heat, skimming any impurities that rise to the surface.
Add the onion studded with cloves, carrot, celery, and bouquet garni. Lower the heat and simmer gently for 1.5 to 2 hours, or until the veal is tender. Remove the veal and strain the broth, setting both aside.

Prepare the Roux:

In a separate pan, melt the butter over medium heat. Add the flour and cook, stirring constantly, for 2 minutes without letting it brown.
Gradually whisk in the reserved broth until the mixture is smooth and thickened. Simmer for about 10 minutes, stirring occasionally.

Finish the Sauce:

In a bowl, mix the cream and egg yolk. Gradually whisk in some of the hot sauce to temper the mixture. Then, slowly stir this mixture back into the saucepan. Add lemon juice, and season with salt and pepper. Keep the sauce warm, ensuring it does not boil.

Cook Mushrooms and Pearl Onions:

In a skillet, sauté the mushrooms and pearl onions in butter until they are golden.

Combine and Serve:

Add the veal, mushrooms, and onions to the sauce, warming everything through.
Serve hot, garnished with fresh parsley, alongside rice, steamed vegetables, or mashed potatoes.