In a large mixing bowl, whisk together pineapple juice, soy sauce, brown sugar, honey, rice vinegar, sesame oil, garlic, ginger, and optional red pepper flakes.

Place chicken thighs in a resealable bag or shallow dish and pour half of the marinade over. Seal or cover and refrigerate for at least 2 hours.

Preheat grill to medium-high heat. Remove chicken from marinade and grill for 5-7 minutes on each side until internal temperature reaches 165°F (74°C).

Pour reserved marinade into a saucepan, bring to a boil, then simmer for 5 minutes. For thicker sauce, mix cornstarch with water and stir into sauce.

During the last minute of cooking, brush thickened sauce onto chicken. Transfer to a platter and garnish with green onions, sesame seeds, and pineapple.