Toss the breadcrumbs into a small bowl and pour the milk over them. Stir to combine then set aside the mixture until the crumbs have absorbed all the milk.
Place the ground beef into a large bowl and add the grated onion, minced garlic, and egg yolk. Add the Parmesan cheese. Add nutmeg and parsley to the breadcrumb mixture. Add breadcrumb mixture to the meat mixture. Season with salt and pepper and mix well. Form into 8 large meatballs.
Preheat a large heavy skillet over medium-high heat. When it's hot add a splash of olive oil, enough to cover the bottom. Add the meatballs.
Paitently sear them until they are well browned on all sides. Drain off any excess fat.
Add the tomato sauce of your choice and simmer until meatballs have cooked through about 15-20 minutes.
Cook the spaghetti in lots of boiling salted water, drain without rinsing and serve with meatballs.