Slice chicken across the grain into 1/2-inch strips and pat dry. Slice bell peppers and onion into similar-sized strips. Mix the sauce ingredients (except cornstarch slurry) in a bowl and set aside. Combine cornstarch with water in a small cup and set aside.

Heat a large skillet or wok over high heat. Add 1 tablespoon oil, add half the chicken in a single layer and sear 1–2 minutes per side until lightly browned. Remove to a plate and repeat with the remaining chicken.

Add remaining oil to the hot pan. Sauté onion 1 minute, then add garlic and ginger and cook 20–30 seconds. Add bell peppers and stir-fry 2–3 minutes until bright and just tender.

Return chicken to the pan, pour in the sauce, and bring to a simmer. Stir in the cornstarch slurry and cook 1–2 minutes until the sauce thickens and becomes glossy. Turn off heat and drizzle sesame oil.

Garnish with sliced green onions and sesame seeds. Serve immediately over steamed rice or noodles.