Frico Meatball Subs

Name: Frico Meatball Subs
Ingredients: Nonstick cooking spray, for the foil; 1/4 cup panko breadcrumbs; 2 tablespoons milk; 1 pound ground beef (80/20); 1/4 cup finely chopped fresh parsley, plus more for serving; 2 tablespoons freshly grated Pecorino Romano; 1 large egg, lightly beaten; 1/4 small onion, grated; Kosher salt and freshly ground black pepper; 2 tablespoons plus 4 teaspoons freshly grated Parmesan; 2 cloves garlic, grated; One 32-ounce jar marinara sauce; 4 tablespoons unsalted butter; 8 split-top hot dog buns; 16 slices deli-sliced low-moisture mozzarella (about 10 ounces)