Place the beef in a large resealable bag. Add the cornstarch, seal the bag tightly, and shake to coat the beef evenly.
Heat oil in the Instant Pot® using the Sauté setting. Add the beef to the pot and cook, stirring, for 3 minutes. Remove the beef from the pot and set it aside.
Add water to the pot and stir with a whisk to remove the browned bits from the bottom. Stir in the brown sugar, soy sauce, and seasoning mix until well combined. Return the beef to the pot.
Close the lid and set the valve to Seal. Select Pressure Cook or Manual and cook on High Pressure for 7 minutes.
When cooking is complete, quick-release the pressure. Once the pressure is fully released, carefully open the lid (refer to the manufacturer's manual for safe operating instructions).