Season the diced chicken with salt and pepper.
Set the Instant Pot to Sauté and heat the vegetable oil, then add the diced onion and minced garlic and cook until softened, about 2–3 minutes.
Add the seasoned chicken to the pot and stir to combine with the onions and garlic.
Pour in the soy sauce, ketchup, and sprinkle the red pepper flakes; stir to distribute the sauce.
Close the lid, set the valve to sealing, and pressure cook on HIGH for 3 minutes.
Perform a quick release, then carefully remove the lid once the pin drops.
Stir in the sesame oil, sliced green onions, and honey.
Whisk the cornstarch and water in a small bowl until smooth, add the slurry to the Instant Pot, then turn on Sauté and cook, stirring, until the sauce thickens to your liking.
Serve sprinkled with sesame seeds if desired.