Preheat oven to 325 F degrees. Spray a 9x13 baking dish with cooking spray.
Season the chicken with salt and pepper. Sprinkle the cornstarch over the chicken and gently toss making sure each piece of chicken is coated in cornstarch.
Dip a few pieces of the chicken into the egg and gently toss the chicken around making sure they are coated completely in egg.
Heat the oil in a large skillet, start with about 1/4 cup. Add a few pieces of chicken at a time and cook just until lightly golden, 1 to 2 minutes. You will have to do this in batches until done. Transfer the chicken to a paper towel-lined plate and discard excess oil.
To make the sweet and sour sauce whisk all the sauce ingredients together in a large bowl.
Add the chicken pieces to the prepared baking dish and pour about 2/3 to 3/4 of the sauce over the chicken and toss so that each piece is coated in the sauce.
Bake the chicken for 45 minutes, turning it over after 15 minutes.
Garnish with green onions and sesame seeds. Use the remainder of the sauce for dipping.
This dish will last in an airtight container in the fridge for 3 - 4 days, same with the sauce stored separately. If frozen properly in an airtight container you can ensure this dish will last for 3 - 4 months in the freezer.Please keep in mind that nutritional information is a rough estimate and can vary greatly based on products used.