Cook pasta al dente according to package directions, and set aside.
Heat a skillet on medium high heat and add vegetable oil. Sear the beef strips quickly about 1-2 minutes (its ok if they have a little pink they will cook more in the sauce.) and remove to a plate.
Add the butter to the skillet and melt. Add in the onions, mushrooms and garlic and cook about 3-4 minutes. Sprinkle the remaining cornstarch over the vegetables and stir.
sAdd in pasta and beef, garnich with fresh parsley and serve.