In a medium saucepan combine all of the sauce ingredients. Place over medium-high heat and bring to a boil. Reduce heat to a low boil and stir, cooking until sauce is just thickened and translucent, 2 to 3 minutes. Remove from heat and set aside.
For the chicken: Cut the chicken into 1½-inch pieces and season with the kosher salt. In a bowl, toss chicken with the vinegar, then cornstarch and set aside to heat the oil.
In a large, high sided skillet, heat canola oil over medium-high heat. When hot, carefully add the chicken and cook, turning as needed, until chicken is just turning golden and is cooked through, about 4 minutes. Remove chicken to a paper-towel-lined plate.
Drain all but 2 tablespoons oil from the skillet and over medium heat, add the onion and bell peppers. Cook, stirring, until onion is becoming tender and soft, about 6 minutes. Add the pineapple.
Add the reserved sweet and sour sauce to the skillet with the vegetables and then stir in the chicken. Heat through, 30 seconds to a minute and then serve over rice garnished with scallions and sesame seeds.
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Notes
VEGGIES:You can swap some of the bell peppers for other tender vegetables like bok choy, broccoli florets, cauliflower, yellow squash or zucchini. CHICKEN:You can swap the chicken for pork tenderloin or firm tofu. Adjust cooking time for tofu because it won't take as long.