Step 1Cut the meat in slices about 1/3 inch thick, then the slices into strips 2 1/4 inches long. You should have about 4 cups. Set aside.
Step 2Heat the broth in a small saucepan until hot.
Step 3Melt the butter in a large skillet over medium heat. Add the onion and cook until softened but not brown, stirring frequently, about 3 minutes. Increase the heat to medium-high and add the mushrooms. Cook until the mushrooms become tender and start to release their juices, stirring frequently, 5 minutes. Add 6 to 8 grinds of nutmeg and stir. Sprinkle in 1 tablespoon of the flour and cook for 1 minute, stirring occasionally. Do not brown. Add the hot broth and stir until smooth.
Step 4Add the beef to the pan, stir well, and warm just to a simmer, about 3 minutes. Stir the remaining 1 tablespoon of flour into the sour cream, add to the pan and season with salt and pepper to taste. Warm gently over low heat until hot, about 7 minutes. The sauce will thin a little; do not boil or the sour cream will break up. Add more nutmeg, if desired. Spoon over the noodles.