In a medium bowl, whisk together the pineapple juice, soy sauce, brown sugar, honey, rice vinegar, sesame oil, minced garlic, grated ginger, and crushed 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; reserve the remaining half in the refrigerator for later. Seal the bag or cover the dish and refrigerate for at least 2 hours, or up to overnight for deeper flavor.

For the grill: Preheat your grill to medium-high heat (about 375-400°F). Remove chicken from the marinade and discard the used marinade. Grill the chicken for 5-7 minutes per side, or until the internal temperature reaches 165°F and the chicken is nicely charred.

For the skillet: Heat 2 tablespoons of neutral oil in a large skillet over medium heat. Cook the chicken for 6-8 minutes per side until golden brown and cooked through (165°F internal temperature). Let rest for 5 minutes before slicing.

Pour the reserved marinade into a small saucepan. Bring to a boil over medium-high heat, then reduce to a simmer. If you like a thicker sauce, whisk together the cornstarch and water to form a slurry, then stir it into the simmering sauce. Cook for 1-2 minutes until glossy and thickened.

Slice the cooked chicken against the grain. Arrange on a platter, drizzle with the thickened teriyaki sauce, and garnish with chopped green onions, toasted sesame seeds, and optional grilled pineapple. Serve immediately with steamed rice or stir-fried vegetables.