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 marinade and set aside for basting later.
02 - Place the chicken thighs in a large resealable plastic bag or shallow dish. Pour the remaining marinade over the chicken, ensuring each piece is evenly coated. Seal and refrigerate for at least 20 minutes, or up to 2 hours for deeper flavor penetration.
03 - Heat a large skillet or sauté pan 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 a golden-brown crust forms and the internal temperature approaches doneness. Transfer the chicken 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, tossing frequently, until the vegetables are just tender-crisp and lightly caramelized.
05 - Return the seared chicken to the skillet with the vegetables. Pour in the reserved marinade and bring to a gentle simmer. Stir in the cornstarch slurry and continue cooking for 2 to 3 minutes, spooning the sauce over the chicken, until it thickens to a glossy glaze that coats the chicken and vegetables evenly.
06 - Transfer to a serving platter and garnish with sliced green onions and toasted sesame seeds. Serve immediately while hot.