Preheat your oven to 375°F (190°C).

In a large bowl, combine the ground beef (or turkey), chopped spinach, minced garlic, breadcrumbs, Parmesan cheese, egg, salt, and pepper. Mix with your hands until everything is well blended.

Take a handful of the meat mixture, flatten it in your palm, place a cube of mozzarella in the center, and fold the meat around the cheese to roll it into a ball. Repeat until all mixture is used.

Heat a large skillet over medium heat and add a bit of olive oil. Once hot, add the meatballs, making sure not to overcrowd the pan. Sear them for about 5 minutes, turning occasionally until golden brown.

Transfer the seared meatballs to a baking dish and bake in the preheated oven for about 15-20 minutes, until cooked through and the cheese is gooey inside.

Serve hot, straight from the oven, with a side of pasta or a simple salad.