Preheat your oven to 375°F (190°C) and line a baking dish with parchment paper.

In a large mixing bowl, combine the ground beef, chopped fresh spinach, minced garlic, breadcrumbs, and egg. Mix until just combined.

Take a portion of the meat mixture and flatten it in your hand. Place a cube of mozzarella cheese in the center, then fold the sides over and roll it into a ball.

In a skillet, heat olive oil over medium-high heat. Add the formed meatballs and sear for about 3-4 minutes on each side.

Transfer the seared meatballs to the lined baking dish and bake for 20-25 minutes until cooked through.

Check one meatball for doneness by cutting it in half; the cheese should be melty and the beef tender.