Preheat the oven to 375°F (190°C) and grease a large baking dish.
Heat the oil in a skillet over medium heat. Sauté the onions for 4-5 minutes until soft, then add garlic and cook for an additional minute. Remove from heat.
In a large bowl, mix panko, Parmesan, Italian seasoning, salt, and pepper.
In a separate bowl, whisk together buttermilk and eggs, then add to the dry ingredient mixture to create a paste.
Combine ground beef and pork in a large bowl. Add the sautéed onion mixture and breadcrumb mixture. Mix gently until just combined.
Form the mixture into 1.5-inch meatballs and arrange in the prepared baking dish.
Pour tomato sauce over the meatballs, covering them completely.
Cover with foil and bake for 25 minutes. Remove foil, sprinkle with mozzarella, and continue baking for 15 minutes or until cheese melts and is bubbly.
Broil for 2 minutes for a golden cheese top if desired. Let cool slightly, then garnish with basil and serve.
Notes
To make this recipe gluten-free, substitute panko with gluten-free breadcrumbs. Consider adding chopped spinach for an additional nutritional boost.