In a mixing bowl, combine the chopped spinach, ricotta cheese, and half of the mozzarella cheese. Mix well and set aside.
In another large bowl, combine the ground beef, breadcrumbs, egg, minced garlic, onion powder, Italian seasoning, salt, and pepper. Mix until just combined.
Take a handful of the meat mixture and flatten it in the palm of your hand. Place a spoonful of the spinach and cheese mixture in the center, then fold the meat around it to form a ball. Repeat this process until all the meat mixture is used, making about 12-15 meatballs.
Place the meatballs on a baking sheet lined with parchment paper and bake in the preheated oven for about 20-25 minutes, or until they are browned and cooked through.
While the meatballs bake, heat the marinara sauce in a saucepan over medium heat until warmed through.
Once the meatballs are done, remove them from the oven and carefully place them in the warmed marinara sauce. Allow them to simmer for a few minutes to absorb the flavors.
Serve the cheesy spinach stuffed meatballs on a plate, drizzled with additional marinara sauce and topped with the remaining mozzarella and Parmesan cheeses. Garnish with fresh basil leaves.
Notes
For extra flavor, let the meatballs simmer in the marinara sauce for a few minutes.