Place the chicken breasts, 1 at a time, in a Ziploc bag. Pound the chicken to an even thickness, being careful not to puncture the bag. Set aside.

In a medium bowl, combine soy sauce, apple sauce, onion, ginger, garlic, oil, bourbon, BBQ sauce, vinegar, brown sugar, and red pepper flakes.

Reserve 1/3 cup of marinade and transfer the rest to the Ziploc bag with the chicken. Marinate in the refrigerator for at least 30 minutes, ideally overnight.

Preheat grill over medium-high heat. Discard marinade in the bag and grill chicken for 4 minutes per side, until cooked through.

Transfer chicken to a plate, tent with foil, and rest for 5 minutes.

In a small saucepan, combine the reserved marinade and chicken broth. Bring to a boil, then reduce heat and simmer until sauce thickens, about 5-6 minutes.

Slice the chicken, top with sauce, garnish with scallions, and serve.