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

Slice chicken thighs into thin strips and toss in the marinade, ensuring every piece is well coated. Marinate for at least 20 minutes, preferably up to 2 hours for deeper flavors.

Heat a large skillet over medium-high heat and add a drizzle of oil. Once hot, add the marinated chicken in a single layer and cook for 5-7 minutes, turning occasionally until fully cooked and caramelized.

Transfer cooked chicken to a serving plate, sprinkle with toasted sesame seeds and additional chopped green onions, and serve warm with steamed rice or lettuce leaves.