Preheat your oven to 350°F (175°C).
In a large bowl, combine the ground beef, mozzarella cheese, parmesan cheese, egg, minced garlic, Italian seasoning, salt, and pepper. Mix until well blended.
Shape the mixture into meatballs, roughly the size of a golf ball, and place them in a greased baking dish.
Pour the sugar-free marinara sauce over the meatballs, ensuring they are well coated. If you're using spinach, scatter it over the top for added nutrition and color.
Bake in the preheated oven for 25-30 minutes or until the meatballs are cooked through and bubbling.
Serve warm and enjoy your delightful Keto Meatball Casserole!