Preheat your oven to 400°F and line a baking sheet with parchment paper.
Slice the chicken breast into bite-sized pieces. In a bowl, lightly season the chicken with salt and pepper if desired.
In a separate bowl, whisk the egg white until slightly frothy. In another dish, spread the panko breadcrumbs.
Dip each chicken piece first into the egg white, then coat evenly with the panko breadcrumbs.
Arrange the coated chicken pieces on the prepared baking sheet in a single layer.
Bake for 15-18 minutes until the chicken is cooked through and the coating is crispy.
Meanwhile, prepare the sweet and sour sauce: In a small saucepan, combine pineapple juice, apple cider vinegar, minced garlic, fresh ginger, chopped bell pepper, and chopped onion.
Sprinkle in the cornstarch and stir well to ensure there are no lumps. Bring the mixture to a simmer over medium heat and cook for 3-4 minutes until slightly thickened.
Once the chicken is done, remove from the oven and drizzle the sweet and sour sauce over the crispy chicken pieces, or serve the sauce on the side as a dip.
Garnish with extra bell pepper strips if desired and serve immediately.