In a mixing bowl, whisk together fresh pineapple juice, soy sauce, minced garlic, freshly grated ginger, brown sugar, rice vinegar, and sesame oil until the brown sugar has dissolved. Reserve ½ cup of the marinade for basting, and set the remaining aside to marinate the chicken.

Pat the chicken thighs dry and season with salt and pepper. Place into the marinade, ensuring well-coated. Refrigerate for at least 4 hours, ideally overnight.

Remove chicken from the refrigerator 30 minutes before grilling. Preheat grill to medium heat (about 350°F) and brush the grates with oil.

Place marinated chicken thighs skin-side down on the grill. Grill for 10-15 minutes, then flip. Baste with reserved marinade every 10-15 minutes until internal temperature reaches 165°F.

Remove from grill, let rest for 5-10 minutes. Serve garnished with sliced green onions and sesame seeds.