In a medium pot or skillet, melt the butter over medium heat. Add the garlic and cook for about 2 minutes, stirring constantly, taking care not to let the garlic burn. Add the cream cheese, stirring with a whisk until the mixture is smooth, 2-3 minutes. It should come together to form a smooth, creamy paste.
Add the milk gradually, about 1/4 cup at a time, whisking quickly and constantly until each addition is incorporated fully into the sauce. Stir in the Parmesan cheese, pepper, and salt. Stir until the cheese is melted and the sauce is the desired consistency.
Cook for 2-3 minutes for a thicker sauce or remove right after the cheese is melted for a thinner sauce. Yields about 2 cups of sauce. Keep warm.
To assemble the sub:
Preheat oven to 400°F/200°C. Slice the buns in half and lay them face-up on a rimmed baking sheet. Place 3-4 warm meatballs on each bottom half of the buns and top with desired amount of warm sauce, then add sliced cheese. Toast in the oven for 4-5 minutes at 400°F/200°C until the buns are toasted and cheese is melted. Serve.