In a large mixing bowl, combine soy sauce, brown sugar, pineapple juice, ketchup, vegetable oil, minced ginger, minced garlic, rice vinegar, and black pepper. Whisk together until smooth.

Place chicken thighs in a resealable plastic bag or a large dish. Pour the marinade over the chicken, ensuring every piece is well-coated. Seal the bag or cover the dish and refrigerate for at least 1 hour.

Preheat your grill to medium-high heat, around 400°F (200°C). Clean and lightly oil the grill grates to prevent sticking.

Take the marinated chicken thighs out of the fridge, allowing excess marinade to drip off.

Place the chicken thighs skin side down on the grill. Grill for 6-8 minutes before flipping. Cook the other side for another 6-8 minutes until the internal temperature reaches 165°F (75°C).

Brush the chicken with reserved marinade during the last few minutes of grilling for added flavor.

Remove the chicken from the grill and let it rest for about 5 minutes to allow juices to redistribute.

Slice the chicken into pieces and garnish with chopped green onions and sesame seeds before serving.