In a bowl, whisk together the pineapple juice, soy sauce, brown sugar, rice vinegar, vegetable oil, garlic, ginger, and crushed red pepper flakes (if using). This is your marinade.

1. Place the chicken breasts or thighs into a resealable plastic bag or shallow dish. Pour the marinade over the chicken, ensuring that the chicken is fully coated. Seal the bag or cover the dish and refrigerate for at least 30 minutes, or up to 4 hours, for maximum flavor.
2. Preheat your grill to medium-high heat. If using a charcoal grill, make sure the coals are evenly distributed.
3. Remove the chicken from the marinade, discarding any leftover marinade (unless you want to use it as a basting sauce).
4. Grill the chicken for 5-7 minutes per side, or until the internal temperature reaches 165°F (74°C) and the chicken is cooked through, with nice grill marks on the outside.
5. While grilling, if you’d like a thicker glaze for your chicken, pour the leftover marinade into a small saucepan. Bring to a simmer over medium heat and cook for 3-5 minutes until the sauce has thickened. If needed, dissolve 1 tablespoon of cornstarch in 1 tablespoon of water and stir it into the sauce to thicken further.
6. Once the chicken is done, brush it with the thickened marinade and serve hot.
7. Garnish with chopped green onions and sesame seeds for an extra touch of flavor and presentation.