In a medium bowl, combine soy sauce, brown sugar, minced garlic, grated ginger, and sesame oil. Whisk until the sugar dissolves completely.

Add in chopped green onions, black pepper, and chili flakes if desired. Mix well.

Place the sliced chicken in the marinade, ensuring each piece is well-coated. Cover with plastic wrap and marinate for at least 20 minutes at room temperature, or refrigerate for up to 2 hours.

Heat a lightly oiled skillet over medium-high heat. Remove chicken from marinade and add in a single layer. Cook for 5–7 minutes, turning occasionally until cooked through and caramelized.

Once cooked, remove from heat. Garnish with toasted sesame seeds and additional chopped green onions. Serve immediately over rice or in lettuce leaves.