Toast bread slices in toaster until hard and golden brown, about 5 minutes. Remove and cool on wire rack until no longer warm to the touch, about 10 minutes.

Crumble toasted bread slices into a large bowl until reduced to fine crumbs, or place in food processor and process until fine.

Combine chicken, apple, onion, breadcrumbs, egg, soy sauce, and oyster sauce in a bowl until well mixed.

Shape mixture into 4 patties. Place flour on a place, coat patties with flour.

Spray a nonstick skillet with cooking spray, heat to medium. Cook patties until browned on both sides, no longer pink in the center, and juices run clear, about 7 minutes per side. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).