Heat olive oil in a skillet over medium heat. Add spinach and toss until wilted. Stir in garlic and sauté until fragrant. Remove spinach, chop finely, and set aside.
In a large bowl, combine ground beef, ground pork, bread crumbs, eggs, milk, garlic, Parmesan, salt, pepper, and the spinach mixture. Mix until well combined.
Form the meat mixture into balls, pressing a small cube of mozzarella into the center of each.
Heat olive oil in a skillet and pan-fry the meatballs until golden brown on all sides.
Add a few spoonfuls of marinara sauce to the skillet, cover with foil, and bake in a preheated oven at 350°F (175°C) for 20-25 minutes.
Serve over pasta or with a side of bread and extra Parmesan.