Start cooking your rice so it will be ready

Combine cornstarch and water in a medium sized bowl and stir to dissolve. Add the reserved pineapple juice, ketchup, soy sauce, brown sugar, and vinegar to the bowl and stir to combine.

Heat 2-3 teaspoons of the oil in a large nonstick skillet over medium-high heat. Add the chicken and garlic. Stir-fry until the chicken is cooked and no longer pink. Remove the chicken from the pan and cover to keep warm.

Add the remaining oil to the pan. Add the broccoli, bell pepper and onion. Stir-fry over medium-high heat until the vegetables are crisp-tender and the broccoli is bright green. Return the chicken to the pan. Add the pineapple chunks and sauce mixture. Bring to a slow boil and cook until everything is heated through and the sauce has thickened.

Serve over the cooked rice or mix in with the stir fry.