Place the chicken breasts in the Instant Pot.
In a bowl, whisk together pineapple juice, soy sauce, brown sugar, ketchup, garlic, and onion. Pour over the chicken.
Lock the lid and cook on high pressure for 15 minutes.
Quick release the pressure. Remove the chicken, shred it with two forks, and return it to the pot.
Add sliced bell peppers. Stir and let the heat soften them for 5 minutes.
If you prefer a thicker sauce, stir in the cornstarch slurry and cook on sauté for a few minutes until it thickens.
Serve hot over rice, garnished with sesame seeds and green onions.