For the Beef: In the Instant Pot, combine the steak, onion, soy sauce, mirin, water, sugar, ginger, salt, and pepper. Stir well.
For the Rice: In a 6 × 3-inch round heatproof pan, combine the rice, water, oil, and salt. Stir well.
Place a trivet in the pot. Set the pan on the trivet.
Secure the lid on the pot. Close the pressure-release valve. Select MANUAL or PRESSURE COOK and set the pot at HIGH pressure for 5 minutes. At the end of the cooking time, allow the pot to sit undisturbed for 10 minutes, then release any remaining pressure.
Open the lid, and remove the rice.
For Finishing: Select SAUTÉ/Normal on the pot. Add the onion and bell pepper. In a small bowl, mix the water and cornstarch to make a slurry. Stir the slurry into the pot and allow to bubble. Cook, stirring frequently, until thickened, 1 to 2 minutes.
Serve over rice. Sprinkle servings with green onions and sesame seeds.