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 is dissolved.

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 later. Seal the bag or cover the dish, then refrigerate for at least 2 hours, or up to overnight for a deeper flavor.

For grilling: Preheat your grill to medium-high heat (about 375°F to 400°F). Grill the chicken for 5 to 7 minutes per side, until the internal temperature reaches 165°F and the thighs are nicely charred.

For skillet cooking: Heat 1 tablespoon of vegetable or avocado 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. Bring it to a boil over medium-high heat, then reduce the heat to low. If you want a thicker sauce, whisk together the cornstarch and water in a small bowl to make a slurry, then stir it into the simmering sauce. Cook for 1 to 2 minutes, until the sauce turns glossy and thickens slightly.

Slice the cooked chicken, drizzle with the thickened teriyaki sauce, and garnish with sliced green onions, sesame seeds, and optional grilled pineapple. Serve immediately with steamed rice or stir-fried vegetables.