In a medium bowl, mix together brown sugar, ketchup, pineapple juice, soy sauce, Worcestershire sauce, apple cider vinegar, ground ginger, minced garlic, paprika, black pepper, and toasted sesame oil. Whisk until smooth.

Reserve ½ cup of the marinade for basting later. Pour the remaining marinade over the chicken in a resealable bag or dish. Seal and refrigerate for at least 4-6 hours, preferably overnight.

Remove the marinated chicken from the refrigerator 20-30 minutes before grilling to bring to room temperature.

Preheat the grill to medium-high heat (400-425°F) and grease the grates with a paper towel dipped in vegetable oil.

Place the marinated chicken thighs on the grill and sear for about 6 minutes without moving. Flip and cook for another 3 minutes.

Baste with reserved marinade, then cook for an additional 1-2 minutes until the internal temperature reaches 165°F.

Remove from the grill and let the chicken rest for about 10 minutes.

Grill the pineapple rounds on the same grates for approximately 3-4 minutes, turning halfway through.

Slice the chicken and serve alongside the grilled pineapple. Garnish with chopped green onions.