Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Heat a drizzle of olive oil in a skillet over medium heat. Add the fresh spinach and cook for 2-3 minutes, just until wilted. Remove from heat and let it cool slightly before chopping.
In a large bowl, combine the ground meat, breadcrumbs, grated Parmesan cheese, minced garlic, egg, salt, and pepper. Add the chopped spinach and mix well until everything is evenly combined.
Take a small portion of the meat mixture and flatten it in your palm. Place a cube of mozzarella cheese in the center and fold the meat around it, shaping it into a ball. Repeat until all the mixture is used.
Arrange the stuffed meatballs on the prepared baking sheet and drizzle lightly with olive oil. Bake for 20-25 minutes until browned and cooked through.
If desired, warm the marinara sauce and spoon it over the meatballs before serving.
Notes
Store leftovers in an airtight container in the refrigerator for up to 3 days. To reheat, microwave for 1-2 minutes or warm in a skillet.