Add the onions to the pan. If necessary, add approximately one tablespoon of olive oil to the pan. Cook the onions until soft, about 5 minutes. Remove onions to the plate with the meat.
Pour the sherry into the pan and deglaze the pan, scraping up any browned bits. Allow the sherry to reduce for about two minutes.
Remove the pan from the heat. Mix in the sour cream, Worcestershire sauce, salt and pepper, and stir until smooth. Return the pan to very low heat. Do not let the sour cream boil, otherwise it may curdle. Stir in the meat and onions. Sprinkled with the chopped parsley, if using.
Serve the stroganoff on top of the traditional egg noodles or rice or potatoes, as desired.
Notes
I prefer to use really sherry that one would drink, not cooking sherry. In a pinch, you could substitute white wine or chicken broth, although the sherry lends a distinctive taste in this recipe. If you purchase a bottle for this recipe, it will keep for years in a cupboard and may come in handy from time to time!