Prepare the Meatballs: Soak bread slices in milk. Mix ground beef with an egg, wrung-out bread, parsley, garlic, salt, and pepper. Form into balls, inserting mozzarella in the center of each. Coat in flour, then dipped egg mixed with a little milk, and finally breadcrumbs mixed with parmesan. Fry in oil at 180°C until golden brown.
Make the Tomato Sauce: Sauté finely chopped onion, carrot, celery, and fennel in olive oil. Add garlic, tomato concentrate, and deglaze with white wine or water mixed with cider vinegar. Add tomato puree, season with salt and pepper, and simmer for 20-30 minutes.
Combine and Serve: Blend the sauce if desired, add basil or parsley, and mix with cooked spaghetti. Adjust the sauce consistency with pasta cooking water and olive oil to achieve a creamy texture. Serve garnished with parmesan.