Preheat the oven to 400°F (200°C).
In a large bowl, combine Italian-style breadcrumbs, grated Parmesan cheese, and whole milk. Let sit for 5 minutes to soak.
Add the egg, ground beef, ground pork, 2 cloves minced garlic, dried oregano, salt, black pepper, and chopped fresh parsley to the breadcrumb mixture. Mix thoroughly until well combined.
Form the meat mixture into 12 equal-sized meatballs and place them on a baking sheet lined with parchment paper.
Bake the meatballs in the preheated oven for 15 minutes.
Meanwhile, in a small bowl, mix softened butter with 3 cloves minced garlic and 1 tablespoon chopped fresh parsley.
Spread the garlic butter mixture evenly on one side of each slice of Italian bread.
After the meatballs have baked for 15 minutes, remove the baking sheet from the oven.
Place the garlic buttered side of each bread slice down on a separate baking sheet.
Top each bread slice with 3 meatballs, then spoon 1/4 cup marinara sauce evenly over the meatballs.
Sprinkle shredded mozzarella cheese evenly over the sauce and meatballs.
Return the bread with meatballs to the oven and bake for an additional 10 minutes, or until the cheese is melted and bubbly and the bread is toasted.
Remove from oven and let cool for 2 minutes before serving.