In a mixing bowl, combine the low sodium soy sauce, unsweetened apple sauce, finely chopped yellow onion, sesame oil, grated ginger, light brown sugar, crushed garlic, red pepper flakes (if using), and sesame seeds. Stir well until the sugar is dissolved and all ingredients are nicely blended.
Place the cut chicken breasts into the marinade, ensuring that each piece is well-coated. Cover the bowl and let it marinate in the refrigerator for at least 15 minutes, or up to an hour for more flavor.
Preheat a skillet over medium heat. Once hot, add a splash of oil to prevent sticking.
Remove the chicken from the marinade (saving the marinade for later) and place it in the skillet. Cook for about 5-7 minutes on each side, or until the chicken is golden brown and cooked through, with an internal temperature of 165°F (75°C).
Once the chicken is cooked, pour the reserved marinade over the chicken in the skillet. Allow it to simmer for an additional 2-3 minutes, letting the sauce thicken slightly and coat the chicken.
Remove the chicken from the skillet and let it rest for a couple of minutes. Slice it if desired, then serve it topped with sliced scallions and a sprinkle of extra sesame seeds.