Set your 6-quart Instant Pot to the 'Sauté' function on high. Once hot, add the olive oil. Add the diced onion and cook for 2-3 minutes until softened. Add the ground beef and cook, breaking it up with a spoon, until it is fully browned. Drain any excess grease from the pot.
Stir in the minced garlic and sliced mushrooms and cook for another 2 minutes until the garlic is fragrant and the mushrooms have softened slightly. Press 'Cancel' to turn off the Sauté function.
Pour in a splash of the beef broth and use your spoon to scrape any browned bits from the bottom of the pot (deglazing). This is crucial to prevent a 'Burn' warning. Pour in the remaining beef broth, Worcestershire sauce, salt, and pepper. Stir to combine.
Add the uncooked egg noodles to the pot, spreading them in an even layer. Gently press them down with the back of the spoon to submerge them in the liquid as much as possible. Do not stir them into the beef mixture at the bottom.
Secure the lid on the Instant Pot and set the steam release valve to the 'Sealing' position. Select 'Pressure Cook' (or 'Manual') and set the timer for 4 minutes on high pressure.
Once the cooking time is complete, carefully perform a quick release by moving the steam release valve to the 'Venting' position. Once the pin has dropped, open the lid away from your face.
Give the contents of the pot a good stir to combine the noodles and sauce. Let it rest for 2-3 minutes to cool slightly. Stir in the room-temperature sour cream until the sauce is smooth and creamy.
Serve immediately, garnished with fresh chopped parsley.
Notes
Chef's Tips: For the creamiest sauce, ensure your sour cream is at room temperature before stirring it in. Adding cold sour cream to the hot liquid can cause it to curdle. For an even richer sauce, you can substitute 1/4 cup of the sour cream with 2 oz of softened cream cheese. Storage: Store leftovers in an airtight container in the refrigerator for up to 3 days. The noodles will absorb more liquid, so you may need to add a splash of beef broth when reheating.