Make the Marinade: In a medium bowl, whisk together the soy sauce, pineapple juice, brown sugar, ketchup, grated ginger, minced garlic, Worcestershire sauce, apple cider vinegar, red chili flakes, and toasted sesame oil until the sugar is fully dissolved.
Marinate the Chicken: Place the chicken thighs in a large resealable plastic bag or shallow dish. Pour about two-thirds of the marinade over the chicken, ensuring it's fully coated. Reserve the remaining one-third of the marinade in a separate bowl to use later for basting. Seal the bag/cover the dish and refrigerate for at least 2 hours, or ideally overnight for the best flavor.
Preheat Grill: Preheat your grill to medium-high heat (375-400°F / 190-200°C). Lightly oil the grates to prevent sticking.
Grill the Chicken: Remove the chicken from the marinade (discard the used marinade) and place it on the hot grill. Grill for 6-7 minutes per side.
Baste: In the last few minutes of cooking, begin basting the chicken with the reserved, clean marinade. Flip and baste once more until the chicken is glazed and has nice grill marks.
Check Temperature: Cook until the internal temperature reaches 165°F (74°C) for chicken breasts or 170-175°F (77-80°C) for thighs using an instant-read thermometer.
Rest: Transfer the chicken to a clean plate or cutting board and let it rest for 5-10 minutes before serving. This ensures the juices redistribute, keeping the meat moist.
Garnish and Serve: Garnish with sliced green onions and sesame seeds. Serve immediately with your chosen sides.