Put onion, garlic, ginger, beef broth, soy sauce, sesame oil rice vinegar, gochujang, brown sugar and black pepper into the inner pot of the Instant Pot and stir to combine.
Add beef to pot and stir to completely coat meat with sauce. Close Instant Pot, being sure to set the valve to seal. Set for high pressure cooking for 35 minutes.
When the pressurized cooking is done, let the pot naturally depressurize for about 15 minutes. Start cooking your rice at this point.
Mix the cornstarch with 3 tablespoons cold water, stirring until it is completely dissolved. Carefully turn the sealing valve on the lid to vent, to release any remaining pressure. Set the Instant Pot to sauté. Add 3 or 4 spoonsful of the cooking liquid to the cornstarch mixture, one spoonful at a time. This will gradually warm it and keep it from clumping up in the pot. Stir the warmed cornstarch liquid into the pot and let it cook 1 to 2 minutes, stirring frequently, until liquid slightly thickens into a sauce.
Serve the cooked beef in shallow bowls alongside rice. Be sure to spoon some sauce over the meat and the rice. Top with sesame seeds and green onions and serve immediately.