In a large mixing bowl, whisk together the soy sauce, brown sugar, apple cider vinegar, grated ginger, minced garlic, sesame oil, black pepper, and red pepper flakes until the sugar is dissolved completely.
Add the chicken thighs to the marinade, ensuring they are well coated. Cover and refrigerate for at least 2 hours, or preferably overnight for maximum flavor.
Preheat your grill to medium-high heat.
Remove the chicken from the marinade and let any excess drip off. Reserve the marinade for basting.
Place the chicken thighs on the grill, skin side down. Grill for about 6-7 minutes or until the skin is crispy and browned.
Flip the chicken, brush the tops with the reserved marinade, and grill for another 6-8 minutes, or until the internal temperature reaches 165°F (75°C).
While the chicken is grilling, place the pineapple slices on the grill and cook for 2-3 minutes on each side, until caramelized.
Once the chicken is done, remove it from the grill and let it rest for a few minutes before slicing.
Serve the chicken thighs alongside the grilled pineapple, garnished with chopped green onions.