Open the pineapple can and reserve 1/4 cup of the juice; do not drain the remaining pineapple and juice.
Add chicken pieces to the Instant Pot, then pour in the soy sauce, sesame oil, minced garlic, grated ginger, hoisin sauce, brown sugar, red pepper flakes (if using), pineapple chunks with their juice (except the reserved 1/4 cup), and 2 tablespoons pineapple juice or water; stir to combine.
Close the lid and ensure the pressure valve is sealed. Set the Instant Pot to Poultry or Manual on High Pressure for 4 minutes.
When the cook time finishes, allow a 5-minute natural pressure release, then carefully release any remaining pressure and open the lid.
Select the Sauté function on LOW. In a small bowl whisk 1 1/2 tablespoons cornstarch with 3 tablespoons of the reserved pineapple juice until smooth and lump-free.
Stir the cornstarch slurry into the Instant Pot and cook on Sauté, stirring gently, until the sauce thickens, a few minutes.
If you prefer a thicker sauce, mix 1 tablespoon cornstarch with 1 tablespoon reserved juice, add it to the pot, and cook briefly until thickened.
Turn off the Instant Pot and let the chicken rest 5–7 minutes to allow the sauce to set, then serve over rice and garnish with sliced green onions, sesame seeds, and slivered almonds.
