In a large bowl, combine the chicken pieces, soy sauce, sesame oil, cornstarch, and a pinch of salt and pepper. Mix well and let marinate for at least 15 minutes.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken and cook for about 5-7 minutes, stirring frequently, until the chicken is browned and cooked through.
Add the minced garlic, ginger, and red pepper flakes (if using) to the skillet. Stir-fry for an additional 1-2 minutes until fragrant.
Toss in the chopped scallions and cook for another 2 minutes until they are tender but still vibrant green.
Serve the scallion chicken over cooked rice or noodles.