Cook rice according to package instructions. Meanwhile, in a small bowl or measuring cup, whisk together cornstarch, sugar, soy sauce and vinegar. Set aside.

In a large skillet or wok, heat oil over high. Add chicken, bell peppers and green beans and cook, stirring frequently, until vegetables begin to soften, about 5 minutes. Add scallions, ginger and garlic and cook, stirring frequently, until chicken is cooked through and vegetables are tender, about 4 minutes.

Whisk soy sauce mixture, add to skillet, and cook, stirring constantly, until sauce thickens, about 3 minutes. Remove from hear. Fluff rice with a fork. To serve, spoon chicken mixture over rice and top with peanuts if desired.