In a medium bowl, combine soy sauce, brown sugar, minced garlic, grated ginger, sesame oil, chopped green onions, black pepper, and optional chili flakes. Whisk until smooth.

Slice the chicken into thin strips, about 1/4 inch thick. Toss in the marinade and let marinate for at least 20 minutes.

Heat a large skillet over medium-high heat and add a splash of oil. Once hot, add the marinated chicken in a single layer.

Cook for 5-7 minutes, turning occasionally until lightly browned and cooked through.

Serve warm garnished with toasted sesame seeds and additional chopped green onions.