Add vegetable oil to the Instant Pot and select Sauté. Let the oil heat until shimmering.
Brown beef strips in batches for 1 to 2 minutes per side. Do not crowd the pot. Transfer browned beef to a plate.
Add minced garlic to the pot and sauté about 1 minute, stirring constantly.
Add soy sauce, beef broth, brown sugar, and ginger. Scrape up browned bits from the bottom of the pot.
Return beef and any juices to the pot. Lock the lid, set valve to sealing, and cook on high pressure for 12 minutes.
Quick release the pressure carefully, then switch back to Sauté.
Whisk cornstarch with cold water to make a slurry. Stir it into the pot and simmer 1 to 2 minutes, stirring, until the sauce is glossy and thickened.
Stir in green onions and serve immediately over fried rice or steamed rice.
Notes
Slice flank steak against the grain so it stays tender. Brown the beef in batches instead of crowding the pot, or it will steam instead of sear. The sauce is supposed to be sweet-savory; reduce brown sugar to 1/4 cup if you prefer it less sweet. The cook time stays the same if doubling, but use an 8-quart Instant Pot if possible.
Keyword fried rice, instant pot mongolian beef, mongolian beef