1. In a medium bowl, whisk together the soy sauce, pineapple juice, brown sugar, honey, rice vinegar, vegetable oil, sesame oil, minced garlic, grated ginger, crushed red pepper flakes if using, and black pepper until the sugar is dissolved and the mixture looks smooth and glossy.

2. Place the chicken thighs in a large zip-top bag or shallow dish and pour most of the marinade over the chicken, reserving about 1/3 cup in a small container for later basting. Seal the bag, pressing out extra air, and massage the chicken so every piece is well coated.

3. Refrigerate the chicken for at least 30 minutes and up to 4 hours, turning the bag once or twice during marinating so the flavors soak in evenly.

4. When you’re ready to cook, preheat your grill to medium-high heat. Clean and lightly oil the grates so the chicken doesn’t stick. If you’re using a grill pan or broiler, lightly oil those surfaces as well.

5. Remove the chicken from the marinade, letting excess drip back into the bag, and discard the used marinade. Place the thighs on the hot grill and cook for about 5–7 minutes per side, or until the internal temperature reaches 165°F and the outside has nice charred grill marks.

6. While the chicken grills, pour the reserved 1/3 cup of marinade into a small saucepan and bring it to a gentle simmer over medium heat. Cook for 2–3 minutes, stirring often, until slightly thickened into a glossy glaze. Remove from heat.

7. During the last couple of minutes of grilling, brush the thickened glaze over the chicken thighs, turning once to coat both sides. Allow the glaze to bubble and set on the chicken.

8. Transfer the cooked chicken to a serving platter and let it rest for 5 minutes so the juices redistribute.

9. Sprinkle the chicken with sliced green onions and toasted sesame seeds. Serve hot over rice with your favorite vegetable sides, spooning any remaining glaze from the pan over the top.