PLACE the chicken in a medium bowl and sprinkle with the pepper. Add 1 tablespoon of the arrowroot and toss to coat.
ON a 6-quart Instant Pot, select Sauté and adjust to Normal/Medium. Add the avocado oil. When it’s hot, add half the chicken and cook, stirring once, until golden brown, about 6 minutes. Transfer to a plate. Repeat with the remaining chicken. Select Cancel.
MEANWHILE, in a small bowl, whisk together the broth, coconut aminos, vinegar, sesame oil, fish sauce, and remaining 1 teaspoon arrowroot until the arrowroot is dissolved. Add red pepper flakes (if using).
RETURN all of the chicken to the pot. Pour the sauce over the chicken. Lock the lid in place. Select Manual and cook for 6 minutes. Use quick release. Stir the chicken. Let stand until slightly thickened, 5 to 10 minutes. Meanwhile, cook the broccoli according to package directions.
SERVE the chicken over the broccoli. Sprinkle with the sesame seeds and green onions.
Tips: To steam fresh broccoli florets, combine in a microwave-safe bowl with 3 tablespoons water. Cover and microwave on high for 3 minutes or until crisp-tender. To toast sesame seeds, heat in a skillet over medium heat, stirring, until fragrant and lightly browned, about 2 minutes.