In a bowl, combine the chicken pieces with soy sauce and cornstarch. Mix well and let marinate for at least 15 minutes.
Heat vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken and cook until browned and cooked through, about 5-7 minutes. Remove the chicken from the skillet and set aside.
In the same skillet, add minced garlic and ginger. Stir-fry for about 30 seconds until fragrant.
Add chicken broth, rice vinegar, sugar, sesame oil, and crushed red pepper flakes (if using) to the skillet. Stir well to combine.
Return the cooked chicken to the skillet and add the sliced bell peppers and broccoli florets. Stir-fry for another 3-5 minutes until the vegetables are tender-crisp.
Serve hot over cooked rice and garnish with chopped green onions.