In a large measuring cup or small bowl, combine the sauce ingredients (soy sauce, pineapple juice, brown sugar, rice vinegar, sesame oil, ginger, garlic, and red pepper flakes). Set aside.
In a large skillet, heat 1 tablespoon of oil over medium high heat. Season the chicken lightly with salt and cook until golden and cooked through, about 5 minutes. Transfer to a plate.
Add another tablespoon of oil to the pan. Add red pepper, broccoli, pineapple chunks, and green onions, and cook until vegetables are crisp-tender, 3-4 minutes. Remove to the plate with the chicken.
Return the pan to medium-low heat and pour in the sauce mixture prepared in step 1. Bring to a bubble.
Whisk together the cornstarch and 2 tablespoons of water and add the mixture to the pan, whisking constantly, until it has thickened to a syrupy consistency.
Add the chicken, pineapple and vegetables back to the pan and stir to combine with the sauce and warm.
Sprinkle with sesame seeds and green onions if desired and serve over rice.