Preheat the oven to 375°F (190°C).

Prepare the meatball mixture: In a large bowl, combine the ground beef, chopped spinach, minced garlic, breadcrumbs, grated Parmesan, milk, egg, oregano, salt, and pepper. Mix until well combined.

Form the meatballs: Take about 2 tablespoons of the meat mixture and flatten it into a small patty in the palm of your hand. Place a cube of mozzarella cheese in the center and wrap the meat around it, rolling into a ball. Repeat with the remaining mixture and cheese.

Cook the meatballs: Heat olive oil in a large skillet over medium heat. Brown the meatballs in batches, turning them to ensure all sides are browned, about 5-6 minutes per batch.

Finish in the oven: Transfer the browned meatballs to a baking dish and bake in the preheated oven for 10-12 minutes, or until fully cooked and the cheese is melted inside (internal temperature should reach 165°F or 75°C).

Serve: Optionally, serve the meatballs with marinara sauce on the side for dipping.