Place Instant Pot (IP) on Sauté mode and allow it to heat up. It will display "HOT" when it is ready.
While your IP is heating up, season the sliced flank steak with salt and pepper. Once your IP is ready, add the sesame oil to the pot and then place the steak inside the pot and brown each side. When complete place steak to the side. (you may have to do this in two small batches to ensure browning)
Add onion and garlic to IP and cook until translucent
While the onion and garlic cook, combine soy sauce, brown sugar, and beef stock together and pour it into the pot. Scrape the brown bits from the bottom of the pot to prevent the burn notice from appearing.
Place steak back into the pot and pressure cook on high for 12 minutes then quick release the pressure
Remove the steak and place the trivet and steam rack into the pot. Add the frozen broccoli into the steam rack and pressure cook on high for 0 minutes, quick release when finished and place broccoli to the side.
Place the IP in Sauté mode and mix together the cornstarch and water. Pour mixture into the pot and stir to thicken the sauce to your desired thickness. Add in the steak and broccoli and mix together. (If needed add equal parts of cornstarch and water to increase the thickness of the sauce)
Serve over rice or noodles.