Heat up your pressure cooker: press Sauté -> click on the Adjust button -> select More to get the Sauté More function, which means that the food will be sautéed over medium-high heat. Wait for the Instant Pot indicator to read HOT.
Add sliced beef to a large ziplock bag, add 1 tablespoon cornstarch and shake well to coat it evenly.
Add the oil to the hot Instant Pot, once the oil is hot, add the beef and sauté for 2-3 minutes, stirring a few times. If needed brown the beef in batches, you don't want to add too much as it will start releasing juice and it won't brown well.
Note: You can also skip browning the beef and just add it to the pot!
If bits of beef stock to the pot, add 1/2 cup water and deglaze the pot. Using a wooden spoon scrape the bottom of the pot. You can discard that liquid if wanted.
Add the rest of the ingredients:
Add the rest of the ingredients to the pot: minced garlic, minced ginger, lite soy sauce, brown sugar, water, rice vinegar, and red pepper flakes. You can add less sugar, based on your taste and preference.
Stir well until all the ingredients are combined and coated in sauce.
Close lid and pressure cook at High Pressure for 8 minutes + 10 minutes Natural Release. Turn off the heat. Release the remaining pressure manually. Open the lid.
Thicken the Sauce:
Make the cornstarch slurry, in a small bowl mix cornstarch with water until fully combined.
With the Instant Pot on the Sauté function, add the slurry to the pot, stir to combine, and cook for 2-3 minutes on Sauté, stirring occasionally, until the sauce thickens.
Turn off the Instant Pot and let the Mongolian Beef sit for 8-10 minutes before serving, in this time the sauce will settle and thicken more.
Serve over rice and garnish with fresh chopped green onions and sesame seeds.