In a bowl, combine the corn starch, salt and pepper. Add the pieces of chicken and toss to coat evenly on all sides;
Heat a few tablespoons of your choice of cooking fat in a large wok or skillet set over high heat. When the pan is nice and hot, add the chicken and cook until nice and slightly golden on all sides, about 5 minutes.
Meanwhile, make the sauce: combine all the ingredients together in a glass measuring cup and whisk until well combined. As soon as the chicken is cooked to your liking, add the onions and bell peppers, stir and cook for 3 to 5 minutes, until slightly softened.
Add the pineapple, pour the sauce in, stir well and simmer for a minute or two, until the sauce is thickened and pineapple is warmed all the way through.
Garnish with sliced green onions and black sesame seed and serve without delay with a side of cooked white rice, if desired.