Marinate chicken. Toss thighs with soy sauce, rice wine, and 1.5 tsp cornstarch. Rest 10 minutes.
Whisk sauce. Combine broth, hoisin, vinegar, brown sugar, 1 tbsp cornstarch, and water. Stir in sesame oil and flakes.
Heat a wok or large skillet over medium-high heat until hot.
Add 1 tbsp oil. Stir-fry bell peppers until crisp-tender, about 2 minutes. Transfer to a plate.
Add remaining oil. Spread chicken in the pan. Sear 2 minutes, then stir until cooked through, 4–5 minutes.
Stir in garlic and ginger. Cook until fragrant, about 30 seconds.
Return peppers. Pour in the sauce and toss. Cook until glossy and thick, 1–2 minutes.
Fold in green onions. Sprinkle with sesame seeds. Serve hot over rice or noodles.