Thinly slice the steak into thin strips. Set aside.
To a mixing bowl add the soy sauce, vinegar, pepper, sugar, ketchup, hoisin sauce, oyster sauce, and minced garlic. Stir them together until evenly combined.
Add the meat to a large ziplocking bag and pour in the marinade. Seal, and let it sit for 15 minutes. Flip the bag over, then let it sit another 15 minutes.
Make sure the liner is in your Instant Pot, then add the water to it. Add the onions, spreading them out in an even layer.
Pour the meat mixture out onto the onions, but do not stir. Spread the red peppers out over the meat in an even layer.
Add the lid turning to close, and make sure the vent valve is set to the sealed position. Cook for 1 minute on LOW pressure.
When the pot beeps to indicate it's done cooking, let it sit undisturbed for 10 minutes while it naturally releases pressure. After carefully open the vent valve to release any remaining pent up steam.
Open the pot and hit the sautee button. Mix together the cornstarch and water until evenly combined, then pour the slurry into the pot and stir to combine.
Once the sauce has thickened, hit cancel to stop cooking.
Serve the beijing beef over steamed white rice, and enjoy!