Place the chicken breasts, 1 at a time, in a Ziploc bag. Pound the chicken to an even thickness, avoiding punctures, then set aside.

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

Reserve 1/3 cup of the marinade for the sauce. Pour the remaining marinade into the Ziploc bag with the chicken, seal, and marinate in the refrigerator for at least 30 minutes or up to overnight.

Preheat the grill to medium-high heat. Remove chicken from marinade, discarding excess marinade, and grill for 4-6 minutes per side until fully cooked.

Transfer cooked chicken to a plate and let rest for 5 minutes.

Meanwhile, pour the reserved marinade into a small saucepan, add chicken broth, and bring to a boil over medium-high heat. Reduce heat and simmer for 5-6 minutes until slightly thickened.

Slice the chicken and serve topped with the sauce and garnished with sliced scallions.