Add a splash of veg oil to a large pan on high heat. Add the mushrooms and fry until they give up their water and turn brown, about 3 minutes. Remove the mushrooms to a plate.
Add a fresh splash of veg oil to the pan you used for the mushrooms and turn the heat down to medium-high. Add the onion and garlic. Let sweat for 1 minute. Add the beef and fry until cooked, about 5 minutes. While you wait for the beef, get a large pot of water boiling and throw in the noodles.
Return the mushrooms to the pan and add the stock. Let simmer for 5 minutes. Make a slurry by whisking the cornstarch with 1/2 cup of water, then add it also. Stir until the sauce starts to thicken, about 1 or 2 minutes. Whisk in the sour cream and Worcestershire sauce. Add salt and freshly cracked pepper to taste. Your noodles should be close to cooked by this time. Turn the pan down to low while you wait.
Drain the noodles well and add them to the pan. Toss well to coat. Sprinkle a little parsley on top when serving.