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1. In a large mixing bowl, combine the ground beef and ground pork.
2. Add the breadcrumbs, grated Parmesan cheese, chopped parsley, minced garlic, egg, salt, black pepper, and Italian seasoning to the bowl.
3. Using your hands, mix everything together until just combined. Avoid overmixing to keep the meatballs tender.
4. Form the mixture into meatballs about 1 to 1.5 inches in diameter, yielding around 20-24 meatballs.
5. Preheat your oven to 400°F (200°C).
6. Line a baking sheet with parchment paper or lightly grease it.
7. Place the meatballs on the prepared baking sheet, spaced evenly.
8. Bake for 20-25 minutes, or until cooked through with an internal temperature of 160°F (70°C).
9. Remove from the oven and set aside.
10. In a large casserole dish, spread a thin layer of marinara sauce on the bottom.
11. Place the cooked meatballs on top of the marinara sauce in a single layer.
12. Pour the remaining marinara sauce over the meatballs, ensuring they are well-coated.
13. If using optional ingredients (black olives, green bell pepper, onion), sprinkle them over the top.
14. Sprinkle the shredded mozzarella evenly over the casserole.
15. Cover the casserole dish with aluminum foil to retain moisture.
16. Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
17. Remove the foil and bake for an additional 5-10 minutes for a golden top.
18. Let it cool slightly before serving. Garnish with fresh basil leaves if desired.

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