For the meatballs: Preheat the oven to 400°F. Line a rimmed baking sheet with parchment paper.
In a large bowl, combine the milk, egg, parmesan, breadcrumbs, oregano, salt, and pepper. Add the ground meats and mix gently to avoid overworking the meat. Shape the mixture into small golf ball sized meatballs and arrange them on the prepared baking sheet. You should have 18 to 24 meatballs.
Bake the meatballs until they are cooked through and lightly browned on all sides, about 15 minutes. Remove the meatballs from the oven but leave the oven on.
For the sauce: Meanwhile, in a large sauce pan set over medium heat, heat the oil. Add the onion and garlic and cook until softened and starting to brown, 3 to 5 minutes. Add the tomatoes, breaking them up with a wooden spoon, and bring to a boil, stirring often. Reduce the heat to medium-low and simmer until the sauce has thickened, 15 to 20 minutes. Stir in the basil and season with salt and pepper. (The tomato sauce will keep in an airtight container in the refrigerator for one week, and up to 6 months in the freezer.)
Add the baked meatballs to the simmering sauce. Gently stir to coat and cook for 5 minutes.
For the sandwiches: Arrange the rolls on a baking sheet, cut side up. Put 3 to 4 meatballs on one side of each roll, spoon some of the sauce over the meatballs, and top with the cheeses.
Bake until the cheese is melted, 3 to 5 minutes. Top with fresh basil, close the sandwiches, and serve.
Recipe by The Amiable Cooks at https://theamiablecooks.com/meatball-parmesan-heroes-a-k-a-perfect-meatball-subs/