1. Break the bread into pieces and place in the bowl fitted with the ultrablade knife. Mix at speed 12 for 30 seconds. Place breadcrumbs into a separate bowl.
2. Peel the onion and cut in half. Place onion and parsley in the bowl and mix at speed 12 for 10 seconds. Scrape down the sides of the bowl.
3. Add ground beef, egg yolk, breadcrumbs, Worcestershire sauce, salt and pepper. Mix at speed 12 for 20 seconds or until mixture is well combined. Using your hands, shape mixture into four patties. Cover with plastic wrap and refrigerate until ready to cook.
4. Cook patties on a grill or in a frying pan for about 5 minutes each side or until cooked to desired doneness.