Preheat oven to 350F
In a large bowl, combine ground beef, ground pork, breadcrumbs, Parmesan, milk, eggs, garlic, parsley, salt, pepper, Italian seasoning, and red pepper flakes. Mix gently with your hands until just combined so the meatballs stay tender.
Roll the mixture into balls about 2 1/2 inches wide. You should get around 8 meatballs total.
Heat olive oil in a large skillet over medium-high heat. Add the meatballs in batches and sear until browned on all sides, about 5 to 6 minutes total. Don’t overcrowd the pan.
Remove oil and grease from pan, and add marinara covering meatballs and place in oven at 350F until meatballs are 165F internal.
While meatballs are cooking, prepare bread. Hollow out the middle of the loaf by making a canoe shape and then brush the inside with melted butter, 2 cloves minced garlic, and parsley. Place loaf of bread into oven to crisp up for about 5-7 minutes.
Remove and build the inside by placing slices of provolone down, then meatballs on top, followed by shredded mozzarella, and Italian Seasoning on top.
Place back in hot oven for another 5-7 minutes until cheese is melted. Make sure bread doesn't burn on bottom.
Slice and serve hot.