Slice 1 pound flank steak against the grain it into ¼-inch strips. I find it easier to slice the beef after it has been frozen for 10 minutes or so.
Turn your Instant Pot to saute, add in 1 tablespoon extra virgin olive oil, and let heat for 1 minute. Add in beef and saute until just lightly browned. This should only take about 2 minutes. Turn the Instant Pot OFF by hitting cancel.
In a small bowl, mix together ⅔ cup pineapple juice, ⅓ cup reduced sodium soy sauce, 2 teaspoons freshly grated ginger, 1 tablespoon minced garlic, 1 teaspoon red pepper flakes, ½ tablespoon rice wine vinegar, and ¼ cup brown sugar.
Pour the sauce over the seared beef and using a spatula or wooden spoon, scrape up any browned bits on bottom of the inner pot.
Place the lid on the instant pot and be sure sealed. Using the manual or pressure cook function set the cooking time to 3 minutes on HIGH pressure.
After cook time has elapsed, let the pressure release naturally for at least 10 minutes before doing a quick release of pressure.
To thicken the sauce, turn the Instant Pot back to saute and heat until bubbling. Make a cornstarch slurry by mixing together 2 tablespoons cornstarch with ¼ cup water until fully combined. Whisk the cornstarch slurry into the sauce and allow to cook, stirring constantly, until thickened. This should only take 2-4 minutes.
Turn off the instant pot and serve the beef and sauce over rice. Top with scallions or sesame seeds if desired.
Notes
Searing the Meat: If you want to skip searing the meat, omit the oil from the recipe. Place the sliced beef and sauce in the Instant Pot and cook on high for 4 minutes. Follow instructions for adding cornstarch slurry as written.Sweetener: If you do not want to use brown sugar, honey or maple syrup can be substituted.Sirloin Steak: Feel free to use thinly sliced sirloin steak, but for best results increase the cooking time to 10 minutes on high pressure. Frozen Beef: Be sure your frozen beef is cut into strips and eliminate the step of searing the beef along with the oil from the recipe. Instead, add the frozen sliced beef to the inner pot, along with the Mongolian sauce, and increase the cooking time to 6 minutes on high pressure. Spice Level: Adjust the number of red pepper flakes based on how much spice you like. Leftovers: Store cooled leftovers in an airtight container for up to 3 days in the refrigerator or in a freezer-safe container for up to 3 months. To reheat, defrost overnight in the refrigerator, if needed. Reheat individual servings in a heat-safe bowl, covered with a damp paper towel in 30-second intervals in the microwave until warmed through.