If using cooked beef, slice thin against the grain. If using uncooked beef, freeze for 30 minutes for easy slicing (if time allows). Slice against the grain in thin slices.
Preheat Instant Pot on Saute. Once hot, add olive oil, butter, and onion. Cook for 3-4 minutes, until onion starts softening.
Add mushrooms to Instant Pot, and if using uncooked beef, add to the Instant Pot. Cook 4-6 minutes, until well browned. (You may want to do the beef in batches for a larger amount of beef.)
To Instant Pot, add water, scraping the bottom to deglaze. Add onion soup mix, flour, beef base, garlic powder, and black pepper. Stir until well combined.
If using cooked beef, add to Instant Pot, along with egg noodles and beef broth. Make sure noodles are covered.
Set Instant Pot to Manual, High for 5 minutes (assumes egg noodles have a 10 minute cook time. For 8-9 minute cook time for the noodles, set Instant Pot for 4 minutes.).
Once complete, natural release for 5 minutes, then release pressure. Open Instant Pot, and carefully stir until well combined. Gently stir in sour cream and serve.