01 - Combine chicken pieces with salt and pepper in a bowl. Toss with 2 tablespoons cornstarch until all pieces are evenly coated.
02 - Heat 1 tablespoon vegetable oil in a large skillet or wok over medium-high heat. Add half the chicken and cook for 3 to 4 minutes per side until golden and cooked through. Remove and set aside. Repeat with remaining oil and chicken.
03 - In the same skillet, add minced garlic and grated ginger. Sauté for 30 seconds until fragrant, stirring constantly to prevent burning.
04 - Pour in pineapple juice, soy sauce, brown sugar, rice vinegar, and ketchup. Stir thoroughly to dissolve sugar and combine flavors. Bring to a gentle simmer.
05 - Stir in diced red bell pepper and pineapple chunks. Let simmer for 3 to 4 minutes until vegetables begin to soften and pineapple releases juices.
06 - Return cooked chicken to the pan. Whisk cornstarch slurry again and stir into the sauce. Cook for 2 to 3 minutes, stirring continuously, until sauce thickens and coats the chicken evenly.
07 - Remove from heat immediately. Sprinkle sliced green onions and sesame seeds over the top before serving.