Add the beef, onion, mushrooms, cream of mushroom soup, and Worcestershire sauce to a slow cooker (such as a Crock-Pot) and season with salt and pepper. Stir, cover, and cook on high for 5 hours.
Meanwhile, cook the egg noodles (or white rice, which I sometimes prefer).
When the stroganoff is done cooking, stir in the sour cream. Serve over the egg noodles and garnish with the chopped parsley.
1 can of condensed cream of mushroom soup comes to about 10.75-ounces.