Cut the chicken into bite-sized pieces and place in a medium bowl.
Add soy sauce and cornstarch to the chicken and mix well. Let it marinate for 15-30 minutes.
Chop the scallions, mince the garlic and ginger, and slice the bell pepper.
Heat vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken and cook for 5-7 minutes until browned.
Add the white parts of the scallions, garlic, ginger, and bell pepper to the skillet. Stir-fry for 3-4 minutes.
Return the chicken to the skillet, add chicken broth, sesame oil, and rice vinegar. Stir and cook for an additional 2-3 minutes.
Season with salt and pepper, stir in the green parts of the scallions, and serve hot over rice or noodles.