1. Preheat the oven to 375°F (190°C) if baking. Meticulously chop the spinach and finely mince the garlic, preparing them for the culinary process.
2. Warm of olive oil in a skillet over medium heat. Sauté the minced garlic until aromatic, approximately 1 minute. Add spinach and cook until completely wilted and moisture dissipates, roughly 3-4 minutes. Enhance with salt and pepper, then transfer to a plate for cooling.
3. In a spacious mixing bowl, amalgamate ground beef, cooled spinach-garlic mixture, breadcrumbs, beaten eggs, milk, Parmesan cheese, salt, and pepper. Gently fold ingredients to maintain meat’s tenderness, avoiding excessive manipulation.
4. Craft uniform meatballs approximately the size of golf balls. Create a small cavity in each meatball’s center, strategically inserting a mozzarella cheese cube. Carefully seal the cheese completely within the meat mixture.
5. Choose cooking method:

Oven Method: Position meatballs on a greased baking sheet. Roast for 20-25 minutes until exterior turns golden brown and interior reaches proper doneness.

Stovetop Method: Heat a thin oil layer in a skillet over medium-high temperature. Sear meatballs, rotating to achieve uniform browning. Reduce heat, cover, and continue cooking for 10 minutes until thoroughly cooked.