Combine beef and pork meat in a large bowl. Add Parmigiano Reggiano, milk, egg, garlic and parsely and mix. Season with salt and freshly ground black pepper to taste. Shape into 2-inches meatballs (around 10-12). Cook them in a frying pan with a drizzle of olive oil to seal the meatballs on both sides. Add Marinara sauce and let it simmer on a low flame until meatballs are ready, around 15-20 minutes Cut the small buns in half and fill with meatballs and sauce.