Pre-heat oven to 350 degrees
In a large bowl, combine ground beef, ground pork, beaten egg, cheese, parsley, salt, and pepper. Mix together until just combined.
In a medium bowl, combine milk and breadcrumbs, garlic, and onion.
Add milk mixture to the meat mixture being careful not to overwork ingredients.
Form into desired size meatball, greasing hands before starting to keep the meat from sticking, and place on a parchment-lined baking sheet.
Press your thumb into the meatball to create a well. Place a cube of cheese into the well and mold the meat over the ball. Roll in the palms of your hands to form an even ball, place on the parchment-lined baking pan. Repeat with the remaining ingredients.
Place the pan in the preheated oven and roast until the outsides are browned and they read 165°F on a meat thermometer (about 25 minutes).
While the meatballs are cooking, combine the butter, garlic salt, and parsley in a small bowl. Brush the butter mixture all over each hoagie bun.
Remove the meatballs from the oven and set aside.
Place 2 slices of provolone cheese on each buttered hoagie bun. Add 3-4 meatballs on top of the cheese. Top with marinara sauce and 2 more slices of provolone cheese.
Place sandwiches on a foil lined baking sheet.
Bake for 5-10 minutes, or until the cheese melts.
Increase oven temperature to broil.
Broil for 2-3 minutes, or until the cheese is hot and bubbly and the crust browns.
Serve with additional sauce for dipping.