Push the saute button on the Instant Pot and allow for the inner pot to become hot. While the pot is getting hot, thinly slice your flank steak and season with salt and pepper.
Add steak into the pot in batches making sure to not crowd the pot. Cook for 2-3 minutes stirring when one side gets browned. Removed seared steak and add in additional meat.
When all steak has been seared and removed, add in chopped onion and allow to saute for about 2 minutes. Add in garlic and ginger and saute an additional 1 minute.
Add beef broth, soy sauce, brown sugar, sesame oil, and red pepper flakes. Stir until brown sugar has dissolved. Once dissolved, press cancel on the Instant Pot.
Add beef back to pot. Close lid and seal pot. Push manual pressure and adjust time to 12 minutes. When done, allow for a 5-7 minute natural pressure release then release any remaining pressure in the pot.
In a small separate bowl, combine the 3 TBS of water and cornstarch by stirring.
Push saute button on pop and allow for the sauce to thicken. After about 1 minute, add the broccoli florets and allow for them to cook to your liking. (You can also cook broccoli in Instant Pot for 2 minutes on manual pressure and do a quick release)