In a small mixing bowl, whisk together all the sauce ingredients until smooth. If using oyster sauce instead of soy sauce, you can omit the sugar. Set aside.
Heat oil in a large wok or skillet over medium-high heat for 2 minutes until the hot oil sizzles. Add chicken and stir fry for 4-5 minutes until browned on all sides.
Stir in onion, garlic, and Thai chili (optional) and cook until tender and fragrant, about 1 minute. Season the chicken mixture with salt and pepper.
Stir in the sauce and toss well to coat. Keep stirring until the sauce thickens to a desired consistency, about 2 minutes.
Add bell pepper and basil. Cover the lid and cook for 1-2 minutes until the basil is wilted and the bell pepper is tender.
Serve immediately with steamed rice and a drizzle with lime juice if desired.
