Slice chicken into thin strips, about 1/4-inch thick. In a medium bowl, whisk together soy sauce, brown sugar, minced garlic, grated ginger, sesame oil, chopped green onions, black pepper, and chili flakes if using. Stir until the sugar completely dissolves and you have a smooth, glossy marinade, about 30 seconds.

Add the chicken to the bowl and toss until every piece is well coated with marinade. Cover and refrigerate for at least 20 minutes, or up to 2 hours for deeper flavor.

Heat a large skillet over medium-high heat and add the neutral oil. Once the pan is hot and shimmering, add the marinated chicken in a single layer without crowding. Let it sear undisturbed for 5 minutes until golden caramelized edges form. If chicken browns too quickly, reduce heat slightly.

Flip the chicken pieces and cook for another 5 minutes until cooked through and internal temperature reaches 165°F. The chicken should be beautifully browned with sticky, caramelized bits.

Transfer to a serving plate, sprinkle with toasted sesame seeds and extra green onions, and serve immediately while hot and juicy.