Soak breadcrumbs in milk for five minutes.
Heat olive oil in large skillet over medium heat.
Sauté onions until soft and translucent, about minutes.
Add garlic and cook for one additional minute.
Remove from heat and let mixture cool completely.
Combine ground beef and pork in mixing bowl.
Add soaked breadcrumbs, beaten egg, and cooled vegetables.
Season with salt, pepper, nutmeg, and parsley.
Mix gently until ingredients are just barely combined.
Form mixture into golf-ball-sized portions using hands.
Heat same skillet over medium-high heat again.
Brown meatballs on all sides, about minutes total.
Add tomato paste and cook for minutes more.
Reduce heat and simmer until cooked through.