Toss the chicken pieces with 1 tbsp cornstarch to lightly coat.
Heat oil in a large skillet or wok over medium-high heat. Add chicken and cook until browned and cooked through, about 6–8 minutes. Remove from the pan and set aside.
In the same skillet, add more oil if needed. Stir-fry bell peppers, garlic, ginger, and pineapple chunks for 2–3 minutes until just tender and fragrant.
In a small bowl, whisk together pineapple juice, soy sauce, rice vinegar, honey, ketchup (if using), and cornstarch.
Return the chicken to the skillet and pour the sauce over everything. Cook, stirring constantly, for 2–3 minutes until the sauce thickens and coats the chicken and veggies.
Serve hot over rice or noodles. Garnish with green onions or sesame seeds if desired.