Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a large mixing bowl, combine the ground meat, chopped spinach, shredded mozzarella, breadcrumbs, grated Parmesan, minced garlic, the egg, oregano, salt, and black pepper. If using fresh basil, add it now as well.
Use your hands to mix the ingredients until just combined. Be careful not to overmix, as that can make the meatballs tough.
Scoop out 1-2 tablespoons of the mixture and roll it into a ball (about 1.5 inches in diameter). Repeat until all the mixture is shaped into meatballs and place them evenly on the prepared baking sheet.
Place the baking sheet in the preheated oven and bake for 18-20 minutes, or until the meatballs are cooked through and browned on the outside.
While the meatballs are baking, heat the marinara sauce in a saucepan over medium heat. Once the meatballs are done, add them to the sauce and simmer for an additional 5-10 minutes to soak in flavor.
Serve the meatballs hot with marinara sauce, garnished with additional shredded mozzarella or fresh basil, if desired.
Notes
Be careful not to overmix the meatball mixture to keep them tender.