Preheat your oven to 400°F (200°C). In a mixing bowl, soak the Italian-seasoned bread crumbs in milk for 5 minutes. Meanwhile, heat olive oil in a skillet and sauté finely diced onions until they're soft and translucent, about 4-5 minutes. Add the sautéed onions to the soaked bread crumbs along with the ground beef, ground pork, egg, and seasonings. Mix gently until just combined to avoid overmixing.

Shape the meat mixture into 12 equal-sized meatballs, about 1.5 inches in diameter. Place the meatballs on a lined baking sheet, ensuring they have space between them for even cooking. Bake in the preheated oven for 23-27 minutes, or until they reach an internal temperature of 165°F (74°C) and are nicely browned.

While the meatballs are baking, divide the pizza dough into 12 equal pieces. Roll each piece into a circle, about 4-5 inches in diameter. In the center of each dough circle, place a slice of mozzarella cheese followed by one cooked meatball. Carefully fold the dough around the filling, pinching the seams to seal tightly.

In a small bowl, mix olive oil, minced garlic, Parmesan cheese, Italian seasoning, and kosher salt for the topping. Brush this mixture over each wrapped meatball. Place the stuffed and brushed dough in a muffin tin. Bake for 18-20 minutes, or until they are golden brown and puffy.

If desired, sprinkle shredded mozzarella cheese on top. Return to the oven for an additional 2-3 minutes, just until the cheese is melted and bubbly. Serve hot, alongside marinara sauce for dipping.