Preheat the oven to 425°F (220°C) and spray a baking sheet with non-stick spray.
Season the chicken cubes with kosher salt and ground black pepper.
Set up a breading station: place the whole-wheat flour in one bowl, beaten egg whites in a second bowl, and whole-wheat panko in a third bowl.
Dredge each chicken cube in flour, dip into the egg whites, then coat evenly with panko breadcrumbs; shake off excess and arrange in a single layer on the prepared baking sheet.
Bake the breaded chicken for about 10 minutes, turning once halfway, until the exterior is crispy (the chicken may not be fully cooked through).
Meanwhile, whisk together pineapple juice, honey, low-sodium soy sauce, cornstarch, and minced garlic in a medium bowl until smooth.
In a large mixing bowl, combine the diced red bell pepper, green bell pepper, red onion, fresh diced pineapple, and the partially baked chicken.
Pour the sauce over the chicken and vegetables and toss gently to coat everything evenly.
Reduce oven temperature to 325°F (165°C) and spray a 9x13-inch casserole dish with non-stick spray.
Transfer the sauced chicken and vegetables to the prepared casserole dish and bake uncovered for 30 to 45 minutes, stirring every 5–6 minutes to ensure even cooking and coating.
Remove from the oven and let rest 5 minutes, then sprinkle with chopped parsley and serve.
