01 - In a mixing bowl, whisk together the soy sauce, pineapple juice, brown sugar, ketchup, minced garlic, grated ginger, and rice vinegar until well combined. Reserve 1/4 cup of the mixture for basting later.
02 - Place the chicken thighs in a large resealable plastic bag or shallow dish. Pour the remaining marinade over the chicken, seal or cover, and refrigerate for at least 20 minutes or up to 2 hours for deeper flavor.
03 - Heat a large skillet over medium-high heat. Remove the chicken from the marinade, discarding any used marinade. Sear the thighs for 4 to 5 minutes per side until golden brown and nearly cooked through. Transfer to a plate and set aside.
04 - In the same skillet, add the sliced red and yellow bell peppers, red onion, and fresh pineapple chunks. Stir-fry for 4 to 5 minutes until the vegetables are just tender and lightly caramelized.
05 - Return the seared chicken to the skillet. Pour in the reserved marinade and bring to a gentle simmer. Stir in the cornstarch slurry and cook for 2 to 3 minutes, turning the chicken occasionally, until the sauce thickens and coats everything evenly.
06 - Transfer to a serving platter, garnish with sliced green onions and toasted sesame seeds, and serve immediately while hot.