Heat the butter in a large skillet over medium heat.
Add sliced onions and cook until caramelized, about 15-20 minutes, stirring frequently.
Stir in the minced garlic and cook for an additional minute.
Sprinkle flour over the onions and garlic; stir well to combine.
Slowly pour in the beef broth, scraping up any browned bits from the bottom of the skillet.
Incorporate Worcestershire sauce and bring to a simmer.
Add the beef strips and continue to simmer until the meat is tender, about 5-7 minutes.
Stir in the sour cream and cook until heated through; do not boil.
Season with salt and pepper to taste.
Serve over cooked egg noodles or pappardelle, garnished with fresh parsley if desired.