Stir breadcrumbs and milk in a bowl and let them soak for 2 minutes.
Add beef, beaten egg, Parmesan, onion, garlic, Italian seasoning, salt, and pepper. Mix gently until just combined.
Roll the mixture into 16 meatballs, about golf-ball size.
Warm olive oil in a large skillet over medium-high heat. Brown meatballs on all sides, about 5–6 minutes.
Pour in marinara and sprinkle red pepper flakes. Reduce heat, cover, and simmer until cooked through, 12–15 minutes.
Set the oven to broil. Split rolls, spread with butter, and broil cut-side up until lightly toasted.
Fill each roll with meatballs and spoon over extra sauce. Top with provolone slices.
Broil until cheese is melted and bubbling. Shower with parsley and serve hot.