- 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.