Preheat oven to 325°F (165°C).
Season chicken cubes with salt and pepper, dredge in cornstarch, and then dip into beaten eggs until coated.
Heat vegetable oil in a large skillet over medium-high heat; sear chicken pieces until golden brown on all sides, then transfer to a 9x13 inch baking dish.
Add the chopped bell peppers, onion, and pineapple chunks to the baking dish with the chicken.
In a medium mixing bowl, whisk together the sugar, ketchup, apple cider vinegar, soy sauce, and garlic salt until smooth.
Pour the sauce mixture evenly over the chicken and vegetables.
Bake for 60 minutes, stirring the mixture every 15 minutes to ensure even coating as the sauce thickens and glazes the ingredients.