Preheat the oven to 375F. Cook the veal on a rack in a roasting pan for 18 minutes per pound. Melt 2 tablespoons butter in a frying pan and cook the onions until softened, about 10 minutes. Make a thick white sauce with the bouquet garni and onion. Stir the white sauce into the mushrooms. Season with nutmeg, salt and pepper. Remove mushrooms from heat and stir in the egg yolks. Slice the roast and spread some of the mushroom mixture onto each slice of veal. Place the veal in a flameproof serving platter. Beat the egg whites until stiff and fold them into the remaining white sauce. Spread sauce over the veal. Place in the oven until the sauce rises slightly and turns golden.
