Preheat the oven to 400°F (200°C).
Pat the chicken thighs dry with paper towels. Season both sides with kosher salt and black pepper.
Heat olive oil in a large oven-safe skillet over medium-high heat. Add the chicken thighs skin-side down and sear until the skin is golden brown and crispy, about 5-6 minutes. Flip and cook for another 3 minutes. Remove the skillet from heat.
In a medium bowl, whisk together pineapple juice, crushed pineapple, honey, soy sauce, minced garlic, grated ginger, and red pepper flakes.
Pour the pineapple sauce over the chicken thighs in the skillet.
Transfer the skillet to the preheated oven and bake for 25 minutes, or until the chicken reaches an internal temperature of 165°F (74°C).
Remove the chicken thighs from the skillet and set aside on a plate.
Place the skillet back on the stove over medium heat. In a small bowl, mix cornstarch and cold water until smooth. Stir the cornstarch slurry into the sauce in the skillet.
Simmer the sauce, stirring frequently, until it thickens, about 3-4 minutes.
Return the chicken thighs to the skillet and spoon the thickened sauce over them.
Garnish with sliced green onions and toasted sesame seeds before serving.