In a large skillet over medium heat, cook onions in olive oil or butter until softened (about 5 minutes).
Add garlic and mushrooms and cook until mushrooms release their moisture and begin to brown.
Push vegetables to the side, add ground beef, and cook until browned, breaking it up as it cooks. Drain excess fat if needed.
Stir in Worcestershire sauce, Dijon mustard, paprika, nutmeg, salt, and pepper. Cook 1–2 minutes.
Add beef broth and stir to deglaze the pan. Let simmer for about 5–7 minutes.
Lower heat and stir in sour cream. Warm gently without boiling.
Scoop out keto portion, then stir in cooked noodles if desired.
Serve and garnish with chopped parsley.