Put the noodles into a heat-resistant mixing bowl and cover them with boiling water. Set aside for 20 minutes, then drain.
In a frying pan, melt the butter, add the onions, and sauté five minutes or so. Add the beef, mushrooms, and noodles. Increase the heat to high and cook for another five minutes, stirring constantly. Add the water and cook another ten minutes over low heat.
In a small bowl, mix together the egg yolks, lemon juice, and sherry. Scoop a few spoonfuls of the meat mixture into the bowl of egg mixture. Turn the contents of the bowl into the skillet. Heat gently while stirring. Serve immediately.