Cut the chicken into bite-sized pieces and season with salt and black pepper.
Sprinkle the cornstarch over the chicken and toss until evenly coated.
In a small bowl, whisk together soy sauce, hoisin sauce, rice vinegar, honey, sesame oil, chicken broth, cornstarch, and optional red pepper flakes.
Heat the oil in a large skillet or wok over medium-high heat.
Add the chicken in a single layer and cook for 3–4 minutes per side, until golden and cooked through. Work in batches if necessary. Transfer to a plate.
Add the sliced bell peppers and onion to the same skillet. Stir-fry for 3–4 minutes until slightly tender but still crisp.
Add the garlic and ginger and cook for 30–60 seconds until fragrant.
Return the cooked chicken to the skillet.
Stir the sauce again, then pour it over the chicken and vegetables.
Cook for 1–2 minutes, stirring continuously, until the sauce thickens and coats the chicken.
Stir in the roasted cashews and half of the green onions.
Cook for another minute, then remove from heat.
Garnish with the remaining green onions and extra cashews before serving.