Place 6 medium boneless, skinless chicken thighs on cutting board. Trim thighs if needed, then cut each chicken thigh into bite-sized chunks.
Season chunks of chicken on all sides with salt and freshly cracked black pepper to taste.
Set Instant Pot to Sauté mode and add 2 tablespoons neutral oil. Heat oil until shimmery, then add seasoned chicken chunks to Instant Pot. Sauté chicken 5 minutes, stirring occasionally. Be careful not to stir too much.
After 5 minutes, add prepared teriyaki sauce to Instant Pot. Stir to coat chicken in sauce, making sure to scrape up any browned bits of food that may be stuck to bottom of Instant Pot insert.
Secure lid on Instant Pot with vent in Sealing position. Set Instant Pot to Manual High Pressure for 2 minutes. Instant Pot will pressurize, then 2-minute cook time will begin.
While chicken cooks, add 2 tablespoons cornstarch and 2 tablespoons cold water to small mixing bowl. Whisk ingredients together vigorously until fully incorporated. Set aside.
When 2-minute cook time ends, immediately Quick Release pressure, then carefully remove Instant Pot lid. Use caution as escaping steam will be very hot.
Using slotted spoon, transfer chicken from Instant Pot to large bowl or plate, leaving as much teriyaki sauce in pot as possible.
Add cornstarch slurry to Instant Pot and whisk well until slurry and teriyaki sauce are fully combined. Set Instant Pot to Sauté mode and bring sauce to boil. Boil sauce 30 to 60 seconds or until thickened, stirring occasionally.
Once sauce has thickened, carefully remove stainless steel insert from Instant Pot. Return chicken to insert and stir to coat in sauce.
Portion chicken into serving bowls with steamed rice. Garnish with thinly sliced green onions and white sesame seeds if desired and serve warm.