01 - Combine chicken pieces with salt and pepper in a bowl. Add cornstarch and toss until evenly coated on all sides.
02 - Heat 1 tbsp oil in a large skillet or wok over medium-high heat. Add half the chicken and cook 3–4 minutes per side until golden and cooked through. Remove and repeat with remaining oil and chicken. Set aside.
03 - Add minced garlic and grated ginger to the same skillet. Sauté for 30 seconds until fragrant, stirring constantly.
04 - Pour in pineapple juice, soy sauce, brown sugar, rice vinegar, and ketchup. Stir thoroughly to dissolve sugar and bring to a gentle simmer.
05 - Stir in diced red bell pepper and pineapple chunks. Simmer for 3–4 minutes until vegetables begin to soften.
06 - Return cooked chicken to the pan. Whisk cornstarch slurry again and stir into sauce. Cook, stirring constantly, until sauce thickens and coats chicken evenly, 2–3 minutes.
07 - Remove from heat immediately. Sprinkle with sliced green onions and sesame seeds just before serving.