In a medium bowl, whisk together the pineapple juice, soy sauce or tamari, brown sugar, ketchup, minced garlic, grated ginger, sesame oil, and chili flakes (if using). Whisk vigorously until the brown sugar is fully dissolved.

Measure out exactly half of the marinade and set it aside in a separate, clean bowl. The other half is for marinating the raw chicken.

Place the boneless skinless chicken thighs in a large zip-top bag or a shallow dish. Pour the non-reserved marinade over the chicken, ensuring everything is fully coated. Press out the air and seal the bag. Marinate in the refrigerator for a minimum of 4 hours, and ideally, up to 12 hours.

Pour the reserved marinade into a small saucepan. Bring it to a boil over medium-high heat. Once boiling, reduce the heat and let it simmer for about 8 to 10 minutes, or until the sauce has reduced by about a third and lightly thickened into a viscous glaze. Set aside and keep warm.

Preheat your grill to medium-high heat (around 375°F to 400°F). Make sure the grates are clean and lightly oiled to prevent sticking.

Place the pineapple rings directly on the grill grates. Grill for 2 to 3 minutes per side, just until they develop nice char marks and soften slightly. Remove and set aside.

Remove the chicken from the marinade and discard the marinade. Place the marinated chicken thighs on the preheated grill. Cook for 5 to 7 minutes on the first side.

Flip the chicken. Using a basting brush, liberally coat the surfaces of the chicken with the reduced Huli Huli glaze. Grill for another 5 to 7 minutes, or until the internal temperature reaches 165°F. Every minute or so during the last few minutes, flip the chicken and baste the other side.

Transfer the chicken to a cutting board and let it rest for 5 to 10 minutes. Slice the chicken across the grain into generous strips.

Place a small, generous mound of warm sticky rice onto each plate. Top the rice with 1-2 grilled pineapple rings. Layer the sliced chicken over the pineapple. Drizzle any remaining glaze over the top and finish with a sprinkle of toasted sesame seeds, sliced green onions, and fresh cilantro. Serve immediately.