Pat chicken dry and trim excess skin or fat.
Warm oil in a wide pot over medium heat.
Sear thighs skin-side down until lightly browned, 4–5 minutes. Flip and cook 2 minutes. Remove briefly.
Whisk soy sauce, 1 cup water, pineapple juice, brown sugar, garlic, ginger, vinegar, sesame oil, pepper, and flakes.
Nestle chicken into the sauce, skin-side up. Bring to a lively simmer.
Cover and simmer gently for 20 minutes.
Uncover, flip chicken, and cook until tender, 10–15 minutes more.
Transfer chicken to a plate. Boil sauce 5–8 minutes to reduce slightly.
If thicker glaze is desired, stir in cornstarch slurry and simmer 1–2 minutes.
Return chicken to the pot and spoon glaze over. Top with green onions and sesame seeds.