Add the ground beef, garlic, parsley, eggs, Parmesan, and 1/4 c. of breadcrumbs to a large bowl. Using clean hands, mix everything together until it's thoroughly combined. Add the milk to soften the breadcrumbs a bit. Mix in up to 1/4 c. more breadcrumbs, until the mixture holds and doesn't stick to your hands. Roll the meat mixture into 2 Tablespoon-sized meatballs.
Heat 1/4-inch of neutral cooking oil in a large skillet over medium heat. Fry the meatballs making sure not to crowd the pan. Roll them around to ensure they get evenly cooked, about 10-15 minutes. Alternatively, you can bake them at 350* F for about 30 minutes. Test a meatball with a meat thermometer - they should reach 165* internal temperature.
Heat the broiler on the oven. To make the sandwiches, begin by combining the butter with the garlic. Split the rolls in half, and spread 1/2 Tablespoon of garlic butter on each side of the hoagie rolls.
Spoon a few tablespoons of marinara sauce on each hoagie roll. Slice 3-4 meatballs per sub, and fan them evenly over the marinara sauce. Sprinkle the mozzarella over the meatballs. Broil for 3-4 minutes, until the cheese is melted and bubbly. Garnish with basil and Parmesan. Enjoy!
You will probably have leftover meatballs. Use them in pasta or soup, or freeze them in an airtight container for up to 2 months!