In a large mixing bowl, combine the ground beef, ground pork, breadcrumbs, egg, milk, parmesan cheese, dried oregano, salt, and pepper.
Form the mixture into small meatballs approximately 1 inch (2.5 cm) in diameter.
Heat olive oil in a large stockpot over medium-high heat. Brown the meatballs in batches until seared on all sides, then remove and set aside.
In the same pot, add the onion, carrots, and celery. Sauté for 6 to 8 minutes until the vegetables begin to soften.
Add the minced garlic and cook for 60 seconds until fragrant.
Pour in the beef stock and diced tomatoes with their juices. Add the bay leaf.
Return the meatballs to the pot and bring the liquid to a gentle boil.
Reduce heat to low, cover, and simmer for 25 to 30 minutes until the meatballs are cooked through and vegetables are tender.
Remove the bay leaf, stir in fresh parsley, and adjust seasoning with salt and pepper before serving.