Preheat oven to 400°.
Place thyme, onion, and bread in a food processor; pulse until fine breadcrumbs measure 1/3 cup. Place breadcrumb mixture in a shallow dish. Place egg whites in a shallow dish. Sprinkle pork with salt and pepper. Dip pork in egg whites; dredge in breadcrumb mixture. Place pork on a broiler pan coated with cooking spray. Bake at 400° for 30 minutes or until a thermometer registers 155°. Let stand 5 minutes. Cut into 1/4-inch-thick slices.