In a small bowl, whisk together soy sauce, brown sugar, and cornstarch until smooth. Set aside.
Heat 1 tablespoon oil in a large skillet or wok over high heat. Add chicken pieces and cook until golden brown and cooked through, about 5-6 minutes. Remove and set aside.
Add remaining oil to the pan. Toss in frozen vegetables and bell pepper, cooking for 4-5 minutes until tender-crisp.
Add garlic and ginger (if using) and cook for 30 seconds until fragrant.
Return chicken to the pan and pour the sauce over everything. Stir well to coat.
Cook for 2-3 minutes, stirring constantly, until sauce thickens and everything is heated through.
Serve immediately over hot cooked rice. Enjoy!