Season the chicken chunks with salt and place into a large zip lock bag. Pour in 1/3 of the cornstarch and seal. Shake well to coat the chicken. Set aside.
In a medium sized bowl, whisk together the cornstarch, 1 cup water, sugar, vinegar, tomato sauce, and soy sauce. Set aside.
In a large pan, over medium heat, heat the oil. Using tongs, remove the chicken pieces from the zip lock bag, shake off any excess cornstarch and place into pan. Brown chicken on all sides and then remove the chicken from the pan and set aside on a plate.
Add the onion and pepper chunks to the pan and cook until slightly softened, about 1-2 minutes. Pour in the sauce mixture and cook for 1 minute. Stir in the canned pineapple chunks and juice. Bring to the boil and then reduce to a simmer. Add the chicken back to the pan and cook together for 5 minutes, or until the chicken is cooked through and the sauce is thickened.
Serve warm over rice and garnish with sliced spring onion.