In a medium bowl, whisk together the pineapple juice, soy sauce, brown sugar, honey, rice vinegar, sesame oil, minced garlic, grated ginger, and red pepper flakes until the sugar dissolves.

Place the chicken thighs in a large resealable plastic bag or a shallow dish. Pour half of the marinade over the chicken, reserving the other half for the sauce. Seal the bag or cover the dish, and refrigerate for at least 2 hours, or overnight for deeper flavor.

For the grill: Preheat your grill to medium-high heat (about 400°F). Grill the chicken for 5 to 7 minutes per side, until the internal temperature reaches 165°F and the outside is nicely charred.

For the skillet: Heat a drizzle of oil in a large skillet over medium heat. Cook the chicken for 6 to 8 minutes per side, until golden brown and cooked through.

Pour the reserved marinade into a small saucepan and bring to a boil over medium-high heat. Reduce the heat to a simmer. In a small bowl, stir together the cornstarch and water to form a slurry, then whisk it into the simmering sauce. Cook for 1 to 2 minutes, until the sauce thickens and becomes glossy.

Slice the cooked chicken into strips, drizzle with the thickened teriyaki sauce, and garnish with sliced green onions, sesame seeds, and grilled pineapple chunks if desired. Serve immediately over steamed rice or alongside stir-fried vegetables.