Heat the Instant Pot on Saute mode (high setting). Add the butter and onions. Saute until softened, about 3 minutes.

Add the mushrooms and saute another 2 minutes. Cancel saute mode.

Add the steak, thyme, salt, pepper, garlic, worcestershire sauce, and beef broth. Stir, scraping the bottom of the pot to get up any browned bits.

Add the egg noodles and press to submerge them as much as possible in the broth.

Secure the lid and set the vent to sealing (if necessary for your model).

Set to cook on high pressure for 5 minutes. When the time is up, allow a 2-minute natural pressure release, then carefully release any remaining pressure. Remove the lid.

Turn on sauté mode again.

Mix together the cornstarch and water. Add to the pot as it boils and stir well to combine. Turn off saute mode.

In a small bowl, mix the sour cream with two tablespoons of the hot liquid from the pot, to temper the sour cream. This keeps it from curdling.

Add the sour cream and parsley to the pot. Stir to incorporate. Taste and adjust seasoning, if necessary.