Place each chicken breast in a zip-top bag and gently pound to an even thickness, about ½ inch thick.
In a medium bowl, whisk together soy sauce, apple sauce, chopped onion, sesame oil, grated ginger, brown sugar, garlic, red pepper flakes (if using), and sesame seeds.
Reserve ¼ cup of the marinade. Pour the remaining marinade over the chicken in the bag. Seal and refrigerate for at least 1 hour.
Preheat grill to medium-high heat.
Remove chicken from marinade and discard used marinade. Grill chicken for 2–3 minutes per side or until it releases easily from the grill and develops grill marks.
Spoon the reserved marinade over each piece and grill for another 2–3 minutes until fully cooked (internal temperature 165°F / 75°C).
Garnish Korean Chicken with extra sesame seeds and sliced scallions before serving.