In a medium bowl, combine chicken pieces, soy sauce, and cornstarch. Mix well and let marinate for at least 30 minutes.
Heat vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken and cook until browned and cooked through, about 5-7 minutes. Remove chicken from the skillet and set aside.
In the same skillet, add the sliced red and green bell peppers, onion, garlic, and ginger. Stir-fry for about 3-4 minutes until the vegetables are tender-crisp.
Return the cooked chicken to the skillet. Add sweet chili sauce, rice vinegar, sesame oil, and red pepper flakes. Stir well to combine and cook for an additional 2-3 minutes until heated through.
Serve hot, garnished with sliced green onions.