1. Peel and chop the onion. Place the onion in the bowl fitted with the ultrablade knife and mix at speed 12 for 10 seconds. Scrape down the sides of the bowl. Add the beef, breadcrumbs, milk, egg yolk, parsley, salt and pepper. Mix again for 15-25 seconds or until desired consistency is reached.
2. Remove the ultrablade knife and mix one last time using your hands. Continue using your hands to form into balls.
3. Heat olive oil in a large nonstick skillet over medium-high heat. Add meatballs. Cook for 6-8 minutes or until browned, turning to brown all sides. Add water, reduce heat and cover. Cook for 2-4 minutes or until thoroughly cooked.
4. Serve immediately.