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

Slice the chicken breast or thighs into thin, even strips, roughly 1/4-inch thick.

Pour the marinade over the sliced chicken and toss well to ensure every piece is coated. Marinate for at least 20 minutes.

Place a skillet over medium-high heat and add a splash of oil.

Carefully add the marinated chicken strips and cook for 5-7 minutes, stirring occasionally.

Transfer the cooked chicken to a serving plate and garnish with toasted sesame seeds and extra green onions.