Season chicken with salt and pepper to taste. Set aside.
Whisk together brown sugar, soy sauce, and water. Set aside.
Press the “sauté” button and add sesame oil to coat the bottom of the pot. When the oil is shimmering, add the onions and additional salt and pepper. Cook, stirring, until soft (about 3-5 minutes).
Stir in the garlic and cook for 30 more seconds.
Add the chicken to the pot and pour the soy sauce mixture over top. Turn off the sauté function and close the Instant Pot (locking the lid).
Press the “Manual” or “Pressure Cook” button and set the cooking time to 10 minutes for chicken thighs or 7 minutes for chicken breasts (if your chicken pieces are bigger than 8 ounces each, you can cut them in half or increase the cooking time to 9 minutes).
When the chicken is finished cooking, release the steam and open the lid right away to prevent over-cooking.
Slice the chicken or shred with forks; return to the pot and stir into the sauce. Taste and season with additional salt and pepper, if necessary. Serve over rice or cauliflower rice. Garnish with sliced green onion, sesame seeds, and red pepper flakes, if desired.