Dice your chicken into 1 inch cubes and assemble a breading station.
Begin dredging your chicken with the cornstarch first, then the egg, then the breadcrumbs. Place breaded chicken cubes on a plate or sheet pan until you’re done.
Combine the brown sugar, soy sauce, vinegar, ketchup, and pineapple juice in a bowl or large measuring cup and stir to combine. Reserve pineapple chunks
Heat 1 cup of oil in a large sauté pan on medium heat. When the oil is very hot, begin frying the chicken. Cook for 5 minutes, or until golden brown, flipping once halfway through the cook time.
Remove the cooked chicken to a sheet pan or plate and if needed, fry remaining chicken.
Pour off all but 2 tablespoons of oil and remove as much excess breadcrumb from the pan as possible.
Add in the onions and peppers and sauté until tender and the onions are beginning to caramelize.
Add pineapple chunks and garlic to the pan and stir, cooking for about 30 seconds to one minute.
Pour sauce over the peppers and stir until the sauce is smooth and beginning to bubble.
Reduce heat to medium low and let simmer until it thickens slightly, 2 minutes or so, then return the chicken to the pan and toss it all to combine. Serve immediately.
Notes
*Cornstarch outside the US is often called corn flour. For US residents, please do not use corn meal.