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

For skillet: Heat 1 tablespoon of vegetable oil in a large cast-iron or nonstick 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 it to a boil over medium-high heat. Reduce the heat to low. In a small bowl, stir together the cornstarch and water to make a slurry. Whisk the slurry into the simmering sauce and cook for 1 to 2 minutes, until the sauce thickens and turns glossy.

Let the chicken rest for 5 minutes, then slice it against the grain. Arrange on a platter, drizzle with the thickened teriyaki sauce, and garnish with sliced green onions, sesame seeds, and grilled pineapple slices if desired. Serve with steamed rice or sautéed vegetables.