In a medium bowl, whisk together the pineapple juice, soy sauce, brown sugar, ketchup, grated ginger, minced garlic, rice vinegar, sesame oil, and red pepper flakes (if using) until the sugar is fully dissolved.

Pour half the glaze into a large zip-lock bag or shallow baking dish. Reserve the remaining half in a separate bowl or jar — this is your basting and finishing sauce. Do NOT let raw chicken contact the reserved sauce.

Add the chicken thighs to the bag or dish with the marinade. Seal tightly and toss to coat every surface. Marinate in the refrigerator for at least 1 hour, or ideally overnight (up to 24 hours) for best flavor.

When ready to cook, remove the chicken from the marinade and pat lightly with paper towels. Discard the used marinade.

Preheat an outdoor grill to medium-high heat (400–425°F). Clean the grates thoroughly and oil them well to prevent sticking.

Place chicken thighs skin-side down on the grill. Cook for 6–7 minutes without moving until the skin is golden and releases naturally from the grates.

Flip the chicken and brush generously with the reserved glaze. Cook for another 6–7 minutes, then flip again and brush once more with glaze. Continue flipping and basting every 3–4 minutes until the chicken is deeply caramelized and cooked through.

Cook until an instant-read thermometer inserted into the thickest part of the thigh (not touching bone) reads 165°F, approximately 25–30 minutes total. Add pineapple rings to the grill during the last 5 minutes and cook until lightly charred on both sides.

Remove chicken from the grill and let it rest for 5 minutes — this allows the juices to redistribute throughout the meat.

Brush one final coat of reserved glaze over the chicken, then garnish with sliced green onions and sesame seeds. Serve immediately with grilled pineapple rings and steamed white rice.