Step 1: In a medium bowl, whisk together the pineapple juice, soy sauce, brown sugar, rice vinegar, grated ginger, minced garlic, and sesame oil until the brown sugar is fully dissolved.
Step 2: Place the chicken thighs in a large resealable bag or a shallow dish. Pour the marinade over the chicken, ensuring all pieces are well coated. Seal the bag or cover the dish and refrigerate for at least 2 hours, or preferably overnight, for maximum flavor.
Step 3: Preheat your grill to medium-high heat (about 375-400°F / 190-200°C) or your oven to 400°F (200°C). Lightly oil the grill grates or a baking dish.
Step 4: Remove chicken from the marinade, discarding any excess marinade (unless you plan to boil it down into a glaze, see notes). Place chicken thighs on the preheated grill or in the baking dish. If grilling, cook for 6-8 minutes per side, until internal temperature reaches 165°F (74°C) and chicken is nicely charred. If baking, bake for 25-30 minutes, flipping halfway through, until cooked through and lightly caramelized.
Step 5: If desired, while chicken cooks, pour the *unused* marinade (or a fresh batch of half the marinade ingredients) into a small saucepan. Bring to a boil over medium-high heat, then reduce heat and simmer for 5-7 minutes, or until slightly thickened into a glaze.
Step 6: Remove cooked chicken from the grill or oven. Brush with the thickened glaze, if using. Let the chicken rest for a few minutes before serving. Garnish with chopped green onions and sesame seeds.