Heat up your beef stock in a quart saucepan and pour over your rinsed mushrooms. Allow the mushrooms to reconstitute for 15 minutes.
Boil your Grand Mere Egg Pasta according to package directions and set aside.
Slice your onion and add it to a 10-inch skillet. Drain your mushrooms and set aside the broth. Add your reconstituted mushrooms to your onions and cook for an additional 10 minutes until the onions are translucent.
In a separate bowl, mix together 1/2 cup beef stock and the flour. Stir until it is combined and then add the Worcestershire sauce, salt and pepper.
Once onions are translucent, add in your flour and stock mixture and cook for an additional 1 minute. Gradually add in 1/2 cup of the stock you set aside to make the mixture loose.
Remove your pan from the heat and add in your sour cream. Stir to combine and taste. Make any desired seasonings now. If the sauce is still too thick, go ahead and add more stock. It should be the consistency of whipping cream.