1In a small bowl, whisk together rice vinegar, ketchup, soy sauce, sugar, and pineapple juice. Set aside.
2In another small bowl, mix cornstarch and water to form a slurry. Set aside.
3Heat vegetable oil in a large skillet over medium-high heat. Season chicken pieces with salt and pepper, then add to the hot skillet. Cook for 5–6 minutes, stirring often, until the chicken is golden brown on all sides and cooked through. Transfer to a plate.
4In the same skillet, add onion and bell peppers. Stir-fry for 3–4 minutes until they begin to soften but still have a slight crunch.
5Add garlic and ginger to the skillet and cook for 30 seconds until fragrant.
6Pour the sauce mixture into the skillet and bring to a gentle simmer. Add the cooked chicken and pineapple chunks back to the skillet.
7Stir the cornstarch slurry and pour it into the sauce, stirring constantly. Cook for 1–2 minutes until the sauce thickens and coats the chicken and vegetables.
8Taste and adjust seasoning with salt and pepper if needed. Serve over steamed white rice and garnish with scallions.