Preheat the oven to 375°F (190°C).
In a large mixing bowl, combine ground beef (or turkey), shredded mozzarella cheese, grated Parmesan cheese, egg, minced garlic, oregano, salt, and black pepper. Mix until fully combined.
Roll the mixture into meatballs, about 1 inch in diameter.
In a pan over medium heat, brown the meatballs on all sides for about 5-7 minutes. Do not cook them all the way through, as they will finish cooking in the oven.
In a separate bowl, mix sugar-free marinara sauce and heavy cream together until well combined.
In a baking dish, spread half of the sauce mixture at the bottom.
Place the browned meatballs over the sauce in an even layer.
Pour the remaining sauce on top of the meatballs.
Sprinkle the shredded mozzarella cheese and grated Parmesan cheese over the top.
Bake in the preheated oven for 20-25 minutes, until the cheese is melted and bubbly, and the meatballs are cooked through.
Remove from the oven and let it cool for a few minutes. Garnish with fresh parsley if desired before serving.