In a bowl, combine pineapple juice, soy sauce, brown sugar, garlic, ginger, and sesame oil. Stir until the sugar dissolves and the mixture is well combined.

Place the chicken thighs in a ziplock bag or shallow dish. Pour the marinade over the chicken, making sure each piece is coated. Seal the bag or cover the dish and refrigerate for at least 4 hours, preferably overnight.

Heat the grill to medium-high. Lightly oil the grill grates to prevent sticking.

Remove the chicken from the marinade and place it on the grill. Grill for 7-8 minutes per side, or until the chicken reaches an internal temperature of 165°F (74°C).

While grilling, baste the chicken with the reserved marinade every 5-7 minutes to develop layers of flavor and a caramelized finish.

Let the chicken rest for 5 minutes after grilling. Garnish with sesame seeds and chopped green onions before serving.