You get the best of both worlds with this sandwich: plenty of juicy meatballs and gooey cheese, but miniature-sized -- so you don't end up with sauce dripped on your shirt. Put a platter on the table and let everyone grab a piece.
Mix the beef, parsley, breadcrumbs, milk, garlic, egg, 1/2 cup Parmesan and 1 teaspoon each salt and pepper together in a large bowl. Form into twelve 2-inch meatballs.
Combine the marinara with 1/2 cup water in a large saucepot and bring to a simmer over medium heat. Gently add the meatballs to the sauce and simmer until cooked through, about 20 minutes.
Preheat the broiler. Split the whole pack of attached dinner rolls in half horizontally. Put the top and bottom halves cut-side up on a baking sheet. Brush with all but 1 tablespoon of the melted butter and broil until toasted, 1 to 3 minutes.
Preheat the oven to 400 degrees F. Spread half the mozzarella on the bottom half of the bread. Top with the meatballs, then 1 cup of the marinara sauce. Sprinkle with 1/2 cup Parmesan and top with the remaining mozzarella. Cover with the top bread piece, then brush the roll tops with the remaining tablespoon melted butter. Sprinkle with the remaining tablespoon Parmesan and the Italian seasoning.
Bake until the bread is toasted and the cheese is melted, 12 to 15 minutes. Serve.
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