Blanch veal: cover with cold water, bring to boil, drain, and rinse (removes impurities).
Return veal to pot with onion, carrots, celery, and bouquet garni. Cover with fresh cold water. Simmer 1 hour until tender.
Cook pearl onions in butter until lightly golden. Add mushrooms, cook until softened. Set aside.
Strain veal stock into a clean pan. Reduce to 300ml. Add cream and reduce again by one-third.
Whisk egg yolks with lemon juice. Remove sauce from heat. Whisk egg mixture into cream sauce slowly.
Return to very low heat, stirring until thickened to coating consistency. Do not boil.
Add veal, onions, and mushrooms. Season with salt, white pepper, and nutmeg. Serve.