Place each chicken breast half, one at a time, in a zip-top bag and pound to an even thickness of about 1/2 inch, being careful not to puncture the bag.

Thinly slice the 2 scallions (white and green parts) and set aside. In a medium bowl, combine 1/4 cup soy sauce, 1/4 cup unsweetened applesauce, 1/4 cup finely chopped yellow onion, 1 tsp sesame oil, 1 tsp grated ginger, 1 tbsp light brown sugar, 2 crushed garlic cloves, 1 tsp red pepper flakes (if using), 1 tsp sesame seeds, and the sliced scallions. Stir until the brown sugar begins to dissolve.

Reserve 1/4 cup of this marinade in a small bowl and set it aside. Add the remaining marinade to the zip-top bag with the chicken, seal the bag, and refrigerate for at least 1 hour (turn the bag once or twice if convenient).

Preheat a grill to medium-high heat. When ready to cook, remove the chicken from the bag and discard the used marinade from the bag. Pat the chicken lightly dry with paper towels.

Place the chicken on the preheated grill and cook 2 to 3 minutes without moving, until it easily releases from the grates and has grill marks. Flip the chicken. Spoon the reserved 1/4 cup of marinade evenly over the chicken pieces.

Continue grilling 2 to 3 minutes more, or until the chicken is cooked through (internal temperature reaches 165°F or juices run clear).

Transfer the chicken to a plate or cutting board and let rest 3–5 minutes. Sprinkle additional sesame seeds and the thinly sliced scallions on top before serving.