In a large bowl, whisk together pineapple juice, soy sauce, brown sugar, ketchup, apple cider vinegar, garlic, ginger, black pepper, and red pepper flakes until well combined.
Add the chicken thighs to the marinade, ensuring they are fully coated. Cover and refrigerate for at least 1 hour, or overnight for best flavor.
Preheat your grill to medium-high heat. Remove the chicken from the marinade, letting excess drip off.
Grill the chicken for about 6-7 minutes per side, or until the internal temperature reaches 165°F and the chicken is nicely charred.
During the last few minutes of grilling, brush some of the reserved marinade over the chicken for extra flavor.
Remove the chicken from the grill and let it rest for 5 minutes before slicing.
Serve the chicken garnished with sesame seeds and sliced green onions.