Heat the oven to 325°F. Lightly grease a 9-by-13-inch baking dish and set it aside while you prepare the chicken.

Cut the chicken breasts into bite-size pieces and season lightly with salt and black pepper. Place the cornstarch in one shallow dish and the beaten eggs in a second shallow dish.

Coat each chicken piece in cornstarch, then dip it into the beaten egg. Turn the pieces gently so the coating covers the chicken without becoming excessively thick.

Heat the oil in a large skillet over medium-high heat. Brown the chicken briefly on all sides, working in batches if needed; the chicken does not need to be cooked through at this stage.

Transfer the browned chicken to the prepared 9-by-13-inch baking dish. Arrange the pieces in an even layer so the sauce can reach as much of the chicken as possible.

Whisk together the sugar, apple-cider vinegar, ketchup, soy sauce, and garlic salt until the mixture is well combined. Pour the sauce evenly over the chicken.

Bake the dish uncovered for about 1 hour. Turn or gently stir the chicken every 15 minutes so the sauce coats all sides and the chicken cooks evenly.

Check that the chicken reaches an internal temperature of 165°F. Serve hot with rice or steamed vegetables.