Strain the pineapple so that you separate out the pineapple chunks and the liquid (juice). You should have about 1 cup of pineapple juice, but don’t worry if you have slightly more or less.
In the bowl of the Instant Pot, combine pineapple juice (keep the pineapple chunks separate), ginger, garlic, ketchup, apple cider vinegar, soy sauce, sugar, toasted sesame oil, red pepper flakes (if using), and chicken. (Note: If you don't want to add the pot-in-pot rice, skip ahead to Step #6.)
Top the chicken and sauce with a 2” rack then with a 7-cup glass bowl. (Be sure that both the rack and bowl are approved for use in the Instant Pot.)
Rinse the rice well.
Fill the bowl with rice and 1 ½ cups water.
Close and lock the lid.
Set the steam valve to Sealing.
Cook on manual / high pressure for 6 minutes.
When done cooking, leave the machine on Warming (most machines automatically revert to this after cooking). Allow pressure to release naturally for 10 minutes. (Tip: I like to set a kitchen timer so that I don’t get distracted and forget.)
Manually release any remaining pressure.
Carefully remove the bowl of rice and rack and set aside, covered, to keep warm.
Turn on the Instant Pot’s saute function to Normal / Medium.
When sauce is simmering, add pineapple chunks and bell peppers to sauce and simmer for 1 minute.
Whisk together cornstarch and 1 Tbsp water.
Pour water-cornstarch mixture into sauce. Simmer, stirring constantly, until sauce thickens, 2 to 3 minutes.
Turn off saute function. Taste sauce. If you’d like it to be spicier, stir in some more red pepper flakes or some hot sauce.
Serve chicken and sauce over rice. Top with chopped green onions.