Place the chicken breasts, 1 at a time, in a Ziploc bag. Pound the chicken to an even thickness, not puncturing the bag. Put breasts in the bag.

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

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

Preheat grill to 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 let rest while you make the sauce.

In a small saucepan, put reserved marinade, add chicken broth, and bring to a boil. Reduce heat and simmer until sauce thickens, about 5-6 minutes.

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