Prepare the stew: Place the veal in a casserole and cover with cold water by 2 inches. Bring to a simmer and cook for 2 minutes. Drain the veal and wash it under cold water to remove all traces of the scum. Rinse the casserole and return the meat.
Pour enough stock into the casserole to cover the veal by 1/2 inch. Bring to a gentle simmer, skimming as necessary for a few minutes. Add the vegetables and bouquet. Taste for seasoning. Cover partially and simmer gently for about 1 1/4 hours, or until the veal is fork tender. It should not be overcooked.
Strain the liquid from the casserole. Rinse the casserole before placing the meat back inside.
Prepare the onions: Using a paring knife, pierce a cross on the root end of the onions. To loosen the skin, plunge the onions into boiling water for 1 minute. Immediately plunge onions into an ice bath. The peel should easily slide off. Place peeled onions in a saucepan with the stock, salt, and butter. Simmer for about 30 minutes.
Arrange the cooked onions over the meat. Reserve the liquid.
Prepare the sauce: In a saucepan, melt the butter. Stir in the flour over low heat until they foam together for 2 minutes. Off heat, whisk in the stock. Bring the sauce to a boil, stirring. Simmer for 10 minutes, frequently skimming off the film which rises to the surface. Fold in the mushrooms and simmer 10 minutes more, skimming. Add salt, pepper, and lemon juice to taste.
The dish can be made ahead up to this point. Film the top of the sauce with 2 spoonfuls of stock to prevent a skin from forming.
About 15 minutes before serving, reheat the stew.
Blend the egg yolks and cream in a mixer. Beat in by spoonfuls 1 cup of the hot sauce. Pour the mixture into the casserole and stir to combine. Heat the stew until the sauce has thickened slightly making sure it does not come to a simmer.
Garnish with parsley and serve with rice.
Recipe courtesy of Sara Moulton