In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, egg, minced garlic, Italian seasoning, salt, and pepper. Mix until just combined.

Roll the mixture into balls about one inch in diameter. If time allows, chill the meatballs in the fridge for 30 minutes before cooking.

Heat the olive oil in a large pot over medium heat. Add the chopped onion, diced carrots, and diced celery. Cook for about five minutes until the vegetables soften.

Pour in the beef broth and diced tomatoes into the pot. Stir and bring to a gentle boil. Adjust salt and pepper as needed before adding the meatballs.

Carefully add the meatballs one at a time to avoid dropping the broth temperature.

Reduce heat to a gentle simmer and cover the pot, leaving a small gap for steam to escape. Cook for 20 to 25 minutes without stirring.

Check meatballs at the 20-minute mark for doneness.

Stir in chopped spinach and cook for about five more minutes until wilted.

Ladle soup into bowls and top with fresh parsley. Serve hot.